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DOCUMENTS 
DIPT- 


Joint  Committee  on  Inductive 
Interference 


TO  THE 


Railroad  Commission  of  the  State 
of  California 


Presenting  the  results  of  an  investigation  of  inductive  interference  by   power  circuits 

with  communication  circuits  and  including  rules   recommended  for 

the  prevention  or  reduction  of  such  interference 


SAN  FRANCISCO,  CALIFORNIA 
SEPTEMBER  28,  1917 


CALIFORNIA  STATE  PRINTING  OFFICE 

SACRAMENTO 

1918 


GIFT  OF 


FINAL  REPORT 


OF  THE 


Joint  Committee  on  Inductive 
Interference 


TO  TNI: 


Railroad  Commission  of  the  State 
of  California 


Presenting  the  results  of  an  investigation    of  inductive    interference   by    power  circuits 

with    communication    circuits    and    including  rules    recommended  for 

the  prevention  or  reduction  of  such  interference 


SAN  FRANCISCO,  CALIFORNIA 
SEPTEMBER  28,  1917 


t'ALll  OUXTA  STATE  PRINTING  OFFICE 

SACIIAMKNTO 

1918 


=;0t>  * 


TABLE  OF  CONTENTS. 


PAGE 
INTRODUCTION    7 

LETTER    OF   TRANSMITTAL 9 

REPORT 11 

PART  ONE. 
History  of   Committee's  Organization    and   Work. 

\'\n •niation   of   Committee^. 12 

I  Vrsonnel    __ 12 

<  >rganizution    13 

Investigations — , 14 

.I'M nances    _: 18 

PART  Two. 
Explanation  of  Problem  and  Summary   of  Results. 

Nature   of   Subject : . 19 

Summary   of  Facts   Established 21 

I.    Primary  Cause 21 

1^,    Interference  to  Telephone  Circuits — Harmonies 21 

::.    Interference  to  Telegraph  Circuits 22 

I.    balanced    and   Residual   Components 22 

5,   C;MIS«-S  and   Remedies  for  Residuals 23 

(i.    factors  Affecting  Intensity  and  Magnitude  of  Induction 25 

(a)   Dimensional  Factors 25 

(&)    Electrical  Factors ^ 25 

7.  Transpositions    26 

8.  Unbalance  of  Communication  Circuits 27 

9.  Transients    and    Abnormal    Conditions 27 

10.  Nonessential    Features    Cause    Greatest    Interference 28 

Guiding    Principles    for    Preventing    Interference 28 

PART  THREE. 
Revised  Rules  Recommended   by  Committee. 

Reasons    for    Revising    Rules _   39 

Text   of   Revised    Rules _  30 

I.  GENERAL   PROVISIONS _  39 

(a)   Applicability  of  Rules 30 

(&)   Co-operation 30 

(c)   Principle  of  Least  Cost 31 

(ri)   Existing  Parallels 31 

(e)   Saving  Clause _  31 


387732 


TABLE   OF   CONTENTS. 

Text    of   Revised   Rules — Continued.                                                                                              PAGE 
II.  DEFINITIONS 31 

(a)  Class  H  Power  Circuit 31 

(b)  Electrically    Connected    31 

(c)  Signal    Circuit 31 

(<7)    Communication    Circuit    32 

(e)  Line 32 

(f)  Parallel 32 

(g)  Configuration    32 

(h)   Transposition    32 

(«)    Barrel    32 

O')    Discontinuity    32 

(fe)    Co-ordination    33 

(Z)     Balanced   and    Residual   Voltages 33 

(m)  Balanced    and    Residual    Currents 33 

III.  LOCATION  OP  LINES 33 

(a)  Avoidance  of  Parallels '. 33 

(b)  Notice  of  Intention 33 

(c)  Distance  Between  Lines 34 

(rf)    Length   of   Parallels — 34 

(e)    Discontinuities —   34 

IV.  DESIGN  AND  CONSTRUCTION  OF  LINES 34 

(a)  General   Requirements   34 

(b)  Arrangement  and  Spacing  of  Power  Conductors 34 

(c)  Transpositions — General    _  35 

(<Y)   Transpositions — Inside   Limits  of   Parallels 30 

V.  DESIGN,  CONSTRUCTION  AND  ARRANGEMENT  OF  APPARATUS 37 

(«)    Quality   and    Suitability 37 

(&)    Rotating  Machinery 38 

(c)    Transformers  and  Their  Connections 38 

(<O    Rectifiers    . —  39 

(e)    Switches    39 

(/)     Fuses 39 

([/)    Electrolytic    Lightning   Arresters _  39 

(h)    Special    Instruments 

(i)     Communication  Apparatus 40 

VI.  OPERATION  AND  MAINTENANCE 40 

(a)  General   Requirements -  40 

(b)  Balance — 40 

(c)  Record  of  Neutral  Current 40 

(d)  Transformers _  40 

(e)  Switching 40 

(/)    Charging  Electrolytic  Lightning  Arresters 40 

(0)   Abnormal   Conditions 41 


TABLE   OP   CONTENTS.  5 

Text   of  Revised  Rules — Continued.  PAGE 

EXHIBIT — ARRANGEMENT  AND  SPACING  OF  POWER  CONDUCTORS 41 

Effect   on    Capacitance   Unbalance 42 

Effect  on  Induction  from  Balanced  Voltages  and  Currents 45 

Recommended  Configurations 46 

Reference 46 

on   Rules . 46 

I.  GENERAL  PROVISIONS 48 

(a)  Applicability   of   Rules 48 

(b)  Co-operation 48 

(c)  Principle  of  Least 48 

(d)  Existing  Parallels 49 

II.  DEFINITIONS 49 

(«)   Class  H  Power  Circuits 49 

(b)  Electrically  Connected — 49 

(c)  Signal  Circuits 49 

(d)  Communication    Circuit 49 

(i)     Barrel 49 

(/)    and   On)   Balanced  and  Residual  Voltages  and  Currents 49 

III.  LOCATION  OF  LINES 50 

(a)  Avoidance    of    Parallels 50 

(b)  Notice  of  Intention 50 

(c)  Distance   Between    Lines 50 

(rf)    Length  of  Parallel 50 

(e)  Discontinuities 50 

IV.  DESIGN  AND  CONSTRUCTION  OF  LINES 51 

(a)  General  Requii-ements 51 

(b)  Arrangement  and  Spacing  of  Power  Conductors 51 

(c)  Transpositions — General 52 

(d)  Transpositions — Inside  Limits  of  Parallels 53 

V.  DESIGN,  CONSTRUCTION  AND  ARRANGEMENT  OF  APPARATUS 53 

(a)  Quality  and   Suitability —  53 

(b)  Rotating   Machinery 54 

(c)  Transformers  and  Their  Connections 54 

(e)  Switches 55 

(f)  Fuses    -- 55 

(#)    Electrolytic  Lightning  Arresters 55 

(h)    Special  Instruments 56 

(i)     Communication  Apparatus 56 


b  TABLE    OF    CONTENTS. 

Comment  on  Rules — Continued.                                                                                                   PAGE 
VI.  OPERATION  AND  MAINTENANCE 56 

(b)    Balance L 56 

(d)  Transformers 56 

(e)  Switching 56 

(f)  Charging  Electrolytic   Lightning  Arresters 56 

(g)  Abnormal   Conditions 57 

Exhibit    57 

APPENDICES. 

APPENDIX       I.     INTERFERENCE  NOT  COVERED  BY  RECOMMENDED  RULES 58 

(a)    Alternating  Current  Railways 58 

(/>)    Constant-Current  Lighting  Circuits 58 

(c)    Power  Circuits  of  Lower  Voltages 59 

(</)    Cables ___  60 

(e)    Telephone  Subscribers'  Circuits. 60 

(/)    Direct  Current  Circuits 61 

(<7)    Other  Cases  of  Interference 61 

APPENDIX     H.     LIST  OF  TECHNICAL  REPORTS 62 

APPENDIX   III.     COMMENTS  ON  REPORT  OF  JULY  7,  1914 67 

APPENDIX    IV.     BIBLIOGRAPHY 69 

Inductive    Interference — General 69 

Interference  from  Electric  Railways 71 

Publications  by  the  Joint  Committee  on  Inductive  Interference-  71 

State  Public  Utility  Commissions — 71 

APPENDIX      V.     ORGANIZATION  CHART__  -72 


INTRODUCTION. 

In  July,  1014,  the  California  Railroad  Commission  published  the  first 
report  of  the  Joint  Committee  on  Inductive  Interference  to  the  Com- 
mission and  arranged  for  free  distribution  of  the  report  to  interested 
engineers  and  other  parties.  We  then  authorized  the  Committee  to 
continue  its  work  and  the  present  final  report  is  the  result. 

Complying  with  requests,  not  only  of  the  members  of  the  Joint  Com- 
mittee, but  also  from  many  other  sources,  we  have  decided  to  publish 
this  final  report  in  the  same  form  as  the  first  report  was  published. 
\Ve  have  also  concluded  to  publish  in  book  form  a  number  of  the  tech- 
nical reports  of  the  Joint  Committee  (see  Appendix  II  of  this  report), 
and  this  publication  is  now  in  the  course  of  printing  and  will  be  sold  at 
cost  by  the  Commission. 

The  rules  proposed  by  the  Committee  have  been  adopted  by  the 
Commission  and  General  Order  No.  52,  superseding  General  Order 
No.  39,  "In  the  Matter  of  the  Construction  and  Operation  of  Power 
and  Communication  Lines  for  the  Prevention  or  Mitigation  of  Induc- 
tive Interference,"  effective  August  1,  1918,  has  been  issued. 

The  task  of  the  Joint  Committee  is  now  completed.  The  Commis- 
sion's appreciation  of  the  work  done  was  expressed  in  a  letter  of 
November  14,  1917,  to  the  individual  members  of  the  Committee,  read- 
ing in  part  as  follows: 

"We  acknowledge  receipt  of  final  report  of  the  Joint  Committee 
on  Inductive  Interference,  dated  September  28,  1917. 

In  receiving  this  final  report  and  in  accepting  the  resignations 
of  the  members  of  the  Joint  Committee,  we  desire  to  express  to 
each  member  of  the  Committee  our  very  sincere  appreciation  for 
the  splendid  work  which  the  Joint  Committee  has  done  during  the 
last  five  years. 

The  work  in  its  complete  form  is  a  monument  to  this  Committee 
of  which  each  member  must  be  justly  proud. 

The  Railroad  Commission  will  give  prompt  consideration  to  the 
recommendations  of  the  Joint  Committee  with  reference  to  certain 
changes  in  General  Order  No.  39.  The  Commission  is  also  taking 
up  the  question  of  the  publication  in  book  form  of  a  number  of  the 
technical  reports  of  the  Joint  Committee. 

As  soon  as  the  matter  of  the  publication  of  the  book  has  been 
finally  determined  upon,  the  Railroad  Commission  will  also  pub- 
lish in  pamphlet  form  the  Joint  Committee's  final  report. 

The  Commission  is  asking  the  Chairman  of  the  Joint  Committee 
to  make  the  necessary  arrangements  so  that  the  records  and  corre- 
spondence files  of  the  Joint  Committee  may  be  transmitted  to  the 
I  fail  road  Commission  for  permanent  custody." 


8  FINAL   REPORT   ON   INDUCTIVE  INTERFERENCE 

Since  the  date  of  that  letter  the  value  of  the  labor  accomplished  by 
the  Joint  Committee  has  become  even  more  apparent,  and  we  take  this 
occasion  to  express  again  our  sincere  appreciation. 

CALIFORNIA  RAILROAD  COMMISSION. 

E.   0.   EDGERTON, 
II.  D.  LOVELAND, 
ALEX   GORDON, 
FRANK  R.  DEVLIN, 

Commissioners. 

San  Francisco,  California, 
September  13,  1918. 


LETTER  OF  TRANSMITTAL. 

September  28,  1917. 
To  the  Railroad  Commission  <>(  UK  SI  alt  of  California: 

GENTLEMEN:  Nearly  five  years  ago  the  Joint  Committee  on  Inductive 
Interference  was  organized  under  the  auspices  of  the  Railroad  Com- 
mission of  the  State  of  California  and  began  its  investigations  with  the 
object  of  developing  a  better  understanding  of  the  subject  of  inductive 
interference  and  particularly  of  acquiring  information  necessary  for 
establishing  regulations  which  could  be  accepted  by  all  interests  con- 
cerned as  being  effective,  comprehensive  and  reasonable. 

In  July,  1914,  the  Joint  Committee  rendered  to  the  Railroad  Com- 
mission a  preliminary  report  containing  an  account  of  its  investigations 
up  to  that  time  and  including  provisional  rules  which  were  recom- 
mended for  immediate  adoption.  These  rules  were  approved  and  are 
embodied  in  your  General  Order  No.  39,  which  is  now  in  effect  in 
California. 

In  its  preliminary  report  the  Joint  Committee  outlined  further 
investigations  which  it  seemed  important  to  make  preparatory  to  putting 
the  rules  into  more  permanent  shape  and  asked  your  authorization  to 
carry  on  these  investigations.  This  request  being  approved,  the  work 
was  resumed  and  much  additional  information  has  since  been  obtained. 
The  past  few  months  have  been  devoted  to  the  preparation  of  a  report 
outlining  briefly  the  main  features  of  all  work  done  by  the  Joint  Com- 
mittee. This  report,  which  is  submitted  herewith,  contains  a  draft  of 
revised  rules  which  are  believed  to  represent  a  substantial  improvement 
over  the  rules  contained  in  the  1914  report.  The  Joint  Committee 
recommends  that  these  revised  rules  be  approved  by  the  Railroad  Com- 
mission and  issued  as  a  new  order  to  supersede  General  Order  No.  39. 

An  attempt  was  made  to  meet  the  urgent  demand  for  information 
defining  the  limiting  relationships  between  power  and  communication 
lines  which  constitute  a  parallel.  It  was  found,  however,  that  sufficient 
information  was  not  at  hand  to  satisfy  all  parties  as  to  a  basis  upon 
which  these  relationships  cguld  be  set  forth  definitely. 

While  the  subject  of  inductive  interference  offers  an  almost  inex- 
haustible field  for  further  investigation,  the  Joint  Committee  feels  that 
the  object  for  which  it  was  formed  has  now  been  substantially 
accomplished  and  it  seems  unnecessary  that  its  work  should  be  further 
prolonged.  The  rules  recommended  will  constitute,  it  is  hoped,  a 
.satisfactory  basis  of  procedure  for  several  years  at  least,  in  dealing 
with  cases  of  inductive  interference,  assuming  a  proper  spirit  of 
co-operation  among  the  companies  concerned.  The  rules  themselves 
definitely  call  for  and  emphasize  the  necessity  for  this  co-operation. 
In  course  of  time,  no  doubt,  the  experience  gained  in  the  application  of 
these  rules,  together  with  advances  in  the  art,  will  make  it  desirable 
that  the  rules  be  changed  in  some  respects,  but  such  changes  are  thought 
to  be  a  matter  of  the  more  or  less  distant  future  and  not  to  require  the 
continuance  of  the  Joint  Committee. 


10  FINAL    REPORT   ON   INDUCTIVE   INTERFERENCE 

From  time  to  time  during  the  course  of  this  investigation  Technical 
Reports  have  been  prepared  setting  forth  in  detail  the  matters  which 
have  been  studied  and  the  results  and  conclusions  derived.  There  are 
in  all  71  of  these  Technical  Reports  which  are  listed  in  Appendix  II  otf 
the  report  herewith  transmitted.  As  they  contain  a  large  amount  of 
valuable  information  and  are  in  fact  the  only  records  of  most  of  the 
data  derived  by  the  Joint  Committee,  it  is  particularly  important  that 
they  be  rendered  available  to  persons  interested.  To  this  end,  the 
Joint  Committee  has  selected  30  of  the  most  valuable  of  these  Technical 
Reports  and  earnestly  recommends  to  the  Railroad  Commission  that 
these  be  printed  in  a  suitable  volume  by  the  State  of  California  and 
offered  for  sale  to  libraries,  colleges,  companies,  societies  and  individuals 
interested.  The  reports  recommended  for  publication  are  so  desig- 
nated in  the  list  given  in  Appendix  II. 

The  official  copies  of  the  Technical  Reports,  other  original  records, 
and  the  correspondence  files  of  the  Committee  are  ready  to  be  placed  in 
the  custody  of  the  Commission. 

In  transmitting  this  report,  which  marks  the  conclusion  of  its  work, 
the  Joint  Committee  desires  to  express  its  satisfaction  at  the  degree 
of  success  which  has  been  reached  in  composing  the  differences  formerly 
existing  in  California  between  the  power  and  communication  interests, 
differences  due  principally  to  lack  of  familiarity  with  the  physical 
aspects  of  this  subject.  While  it  would  be  too  much  to  say  that  there 
is  now  complete  unanimity  of  opinion  on  every  feature,  still  the  Joint 
Committee  has  been  able,  after  careful  consideration  of  all  information 
available,  to  agree  unanimously  upon  the  present  report.  In  the 
opinion  of  the  Committee  the  results  accomplished  emphasize  strongly 
the  superiority  of  a  co-operative  investigation  of  this  kind,  whereby  the 
fundamental  facts  are  ascertained  and  acted  upon,  as  compared  with 
litigation  or  other  methods  of  arbitrary  settlement  without  the  benefits 
derived  from  such  investigation. 

In  conclusion,  the  Joint  Committee  takes  pleasure  in  acknowledging 
that  the  credit  for  what  has  been  accomplished  is  due  primarily  to  the 
Railroad  Commission  of  the  State  of  California  which  has  consistently 
held  to  the  policy  of  a  co-operative  investigation  and  has  cordially  sup- 
ported this  Committee  in  its  work. 

Respectfully  submitted. 

(Signed) 

A.  II.  GRISWOLD,  JAMES  T.  SHAW, 

II.  A.  BARRE,  RICHARD   SACHSE, 

J.  E.  WOODBRIDGE,  F.   EMERSON  HOAR, 

J.  L.  ORD,  ARTHUR  F.  BRIDGE, 

V.  V.  STEVENSON,  A.  L.  WILSON, 

R.  W.  MASTICK,  JOHN  A.  KOONTZ, 

HOWARD.  S.  WARREN,  J.  P.  JOLLYMAN, 

C.  H.  TEMPLE,  P.  M.  DOWNING, 

A.  H.  BABCOCK. 


FINAL  REPORT 

OF    THE 

Joint  Committee  on  Inductive  Interference  to 

the  Railroad  Commission  of  the 

State  of  California 

INTRODUCTION. 

This  report  embodies  the  results  of  an  investigation  by  the  Joint 
Committee  on  Inductive  Interference,  extending  over  a  period  of 
approximately  four  years.  The  task  undertaken  by  this  Committee 
was  a  study  of  the  problem  of  interference  with  communication  (signal) 
circuits  caused  by  the  inductive  effects  of  neighboring  power  (electrical 
supply)  circuits,  including  field  experiments  and  tests  necessary  to 
determine  the  underlying  physical  facts,  and  the  preparation  of  recom- 
mendations to  the  Railroad  Commission  of  the  State  of  California  for 
its  guidance  in  making  rulings  designed  to  prevent  or  mitigate  such 
interference. 

A  previous  report  by  this  Committee  to  'the  Railroad  Commission  was 
rendered  on  July  7,  1914,  embodying  the  results  of  the  first  two  years 
of  the  investigation  and  including  provisional  rules  for  the  prevention 
or  mitigation  of  inductive  interference,  based  on  the  information 
available  at  that  time.  The  rules  therein  recommended  were  adopted 
by  the  Railroad  Commission  in  its  General  Order  No.  39,  effective 
August  20,  1914.  The  previous  report  also  contains  an  outline  of 
further  investigations  which  this  Committee  considered  essential  in 
order  that  additional  information  might  be  acquired  for  amplifying 
and  revising  the  rules  to  make  them  more  definite  and  complete.  These 
further  investigations  have  now  been  carried  out,  as  far  as  practicable, 
Miid  certain  additional  work  directed  toward  the  same  end  has  also 
been  done. 

Having  completed  Hie  field  investigations  and  having  carefully 
analyzed  and  studied  all  information  accumulated,  the  Committee  now 
presents  its  final  report,  including  revised  rules  for  preventing  or 
reducing  inductive  interference.  The  new  rules  are  not  radically  dif- 
ferent in  substance  from,  the  rules  formerly  recommended  and  now  in 
effect  in  California,  but  are  considerably  changed  in  form  and  arrange- 
ment. They  are  more  specific  as  to  certain  of  the  requirements,  more 
complete  in  several  respects,  less  arbitrary  in  setting  physical  limits 
to  be  observed,  more  clearly  expressed  and,  it  is  believed,  better  adapted 
to  the  conditions  of  practical  use. 

This  report,  which  contains  an  account  of  the  Committee's  work 
from  the  beginning,  including  that  covered  by  the  preliminary  report, 
is  divided,  for  convenience,  into  three  parts  as  follows: 

Part  One  gives  a  historical  account  of  the  Committee's  formation 
and  activities. 

Part  Two  presents,  in  nontechnical  language,  so  far  as  possible,  the 
nature  of  the  subject,  a  brief  resume  of  the  principal  facts  established 
or  agreed  upon  by  the  Committee,  and  a  concise  statement  of  the 
physical  principles  underlying  preventive  or  remedial  measures. 
•  Part  Three  contains  the  Committee's  final  recommendations  for  rules, 
together  with  explanations  of  the  same  in  detail, 


12  FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE 

PART  ONE. 

HISTORY  OF  COMMITTEE'S  ORGANIZATION  AND  WORK. 
Formation  of  Committee. 

The  formation  of  the  Joint  Committee  on  Inductive  Interference  was 
the  outgrowth  of  certain  differences  involving  power,  communication 
and  railroad  interests  which  were  brought  to  the  attention  of  the 
Railroad  Commission  of  the  State  of  California.  As  an  alternative  to 
contesting  the  issue  at  that  time  it  was  agreed  by  the  power  and  com- 
munication companies,  with  the  approval  of  the  Commission,  that  a  joint 
investigation  should  be  made  to  obtain  certain  information  essential 
to  a  proper  solution  of  the  difficulties  due  to  inductive  interference. 
The  Commission  desired  that  the  matter  be  thoroughly  investigated 
before  passing  upon  the  general  principles  involved  in  these  difficulties. 
To  this  end  a  general  conference  was  called  to  select  representatives 
to  form  a  "Joint  Committee"  empowered  to  conduct  tests,  experi- 
ments, and  investigations,  the  results  of  which  would  serve  as  a  basis 
of  recommendations  for  rules  and  regulations  to  be  issued  by  the  Com- 
mission, tending  to  minimize  inductive  interference  and  physical 
hazard  arising  from  parallelism  of  different  classes  of  circuits.  This 
conference  was  held  December  16,  1912.  As  a  result  the  Joint  Com- 
mittee on  Inductive  Interference,  representing  the  Railroad  Commis- 
sion and  railroad,  power  and  communication  interests  of  the  state, 
was  organized  and  authorized  by  the  Commission  to  conduct  the  desired 
investigation. 

Personnel. 

The  personnel  of  the  Committee  selected  is  given  below. 

Representing  the  Railroad  Commission: 
Mr.  R.  A.  Thompson,  Chief  Engineer. 
Mr.  A.  R.  Kelley,  Assistant  Engineer. 
Mr.  James  T.  Shaw,  Assistant  Rate  Expert. 
Mr.  F.  Emerson  Hoar,  Assistant  Rate  Expert. 

Representing  Railroad  Interests: 

Mr.  A.  H.  Babcock,  Consulting  Electrical  Engineer,  Southern 
Pacific  Company. 

Representing  Telephone  and  Telegraph  Interests: 

Mr.  A.  H.  Griswold,  Plant  Engineer,  The  Pacific  Telephone  and 
Telegraph  Company. 

Mr.  R.  W.  Gray,  Division  Superintendent,  Western  Union  Tele- 
graph Company. 

Mr.  C.  H.  Temple,  General  Manager,  United  States  Long  Distance 
Telephone  Company. 

Mr.  L.  M.  Ellis,  General  Manager,  Union  Home  Telephone 
Company. 

Representing  Power  Interests: 

Mr.  H.  A.  Barre,  Electrical  Engineer,  Pacific  Light  and  Power 

Corporation. 

Mr.  Louis  Elliott,  Engineer,  Great  Western  Power  Company. 
Mr.  P.  M.  Downing,  Engineer,  Pacific  Gas  and  Electric  Compan5\ 
Mr.  J.  E.  Woodbridge,  Chief  Engineer,  Sierra  and  San  Francisco 
Power  Company. 


FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE  13 

Since  the  formation  of  the  Committee,  through  additions,  resignation 
or  death,  the  personnel  of  the  Committee  has  changed  as  follows: 

Mr.  Louis  Elliott  resigned  and  Mr.  J.  A.  Koontz,  Engineer  of  the 
Great  Western  Power  Company,  was  appointed  in  his  place. 

Mr.  V.  V.  Stevenson,  Electrical  Engineer  of  the  Postal  Telegraph- 
Cable  Company,  and  Mr.  L.  N.  Peart,  General  Superintendent  of  the 
San  Joaquin  Light  and  Power  Company,  were  added  to  the  original 
membership  by  action  of  the  Committee. 

Mr.  R.  A.  Thompson,  Chairman  of  the  Joint  Committee,  resigned. 
Mr.  W.  C.  Earle,  his  successor  as  Chief  Engineer  of  the  Commission, 
was  elected  to  membership  and  chairmanship.  Subsequently  Mr.  Earle 
resigned  and  Mr.  Richard  Sachse,  who  succeeded  Mr.  Earle  as  Chief 
Engineer  of  the  Railroad  Commission,  was  elected  a  member  and 
Chairman  of  the  Committee. 

.Mr.  L.  M.  Ellis  resigned  and  Mr.  R.  W.  Mastick,  Transmission  and 
Protection  Engineer  of  the  Pacific  Telephone  and  Telegraph  Company, 
was  elected  to  membership. 

Mr.  H.  S.  Warren,  Electrical  Engineer  of  the  American  Telephone 
and  Telegraph  Company,  was  elected  to  honorary  membership. 

Mr.  James  T.  Shaw,  Secretary  of  the  Joint  Committee,  resigned. 
Mr.  A.  R.  Kelley  wras  elected  to  the  office  of  Secretary.  The  vacancy  in 
membership  created  by  the  resignation  of  Mr.  Shaw  was  later  filled  by 
the  election  of  Mr.  A.  L.  Wilson,  Assistant  Rate  Expert  of  the  Railroad 
Commission.  Mr.  James  T.  Shaw  was  elected  to  honorary  membership. 

The  death  of  Mr.  L.  N.  Peart  created  a  vacancy  which  was  filled  by 
the  election  of  Mr.  J.  P.  Jollyman,  Engineer  of  Electrical  Construction 
of  the  Pacific  Gas  and  Electric  Company. 

Mr.  A.  R.  Kelley  resigned,  and  Mr.  A.  F.  Bridge,  Assistant  Electrical 
Engineer  of  the  Railroad  Commission,  was  elected  to  membership  and 
to  the  office  of  Secretary. 

Mr.  R.  W.  Gray  resigned  and  Mr.  J.  L.  Ord,  Division  Plant  Superin- 
tendent of  the  Western  Union  Telegraph  Company,  was  elected  to 
membership. 

Organization. 

The  organization  and  personnel  of  the  Joint  Committee  on  Inductive 
Interference  were  approved  by  the  Railroad  Commission  on  January 
6,  1913,  and  the  Committee  thereupon  proceeded  with  its  tests  and 
investigations. 

For  the  more  efficient  conduct  of  its  work  the  Joint  Committee  was 
divided  into  several  subcommittees,  each  assigned  to  and  responsible 
for  certain  branches  of  the  investigation.  The  present  organization  of 
the  Committee  is  given  on  a  chart  presented  as  Appendix  V. 

Early  in  its  work  the  Committee  established  a  field  engineering  staff, 
reporting  to  the  Subcommittee  on  Tests,  to  conduct  the  necessary  tests 
and  investigations.  This  field  staff  was  at  first  composed  of  engineers 
in  the  employ  of  The  Pacific  Telephone  and  Telegraph  Company  and  the 
American  Telephone  and  Telegraph  Company,  and  was  later  augmented 
by  the  addition  of  two  engineers  and  a  stenographer,  engaged  by  the 
Committee.  Since  August,  1914,  the  stenographer  has  been  provided 
by  the  Railroad  Com  mission.  In  November,  1914,  a  third  engineer  was 

8—39628 


14  FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE 

engaged  by  the  Committee  and  three  were  retained  in  its  employ  for 
nearly  a  year. 

The  Committee  wishes  to  express  its  appreciation  of  the  able  manner 
in  which  Mr.  L.  P.  Ferris  has  supervised  the  analytical  and  theoretical 
work. 

Investigations. 

Previous  to  the  formation  of  the  Joint  Committee  in  December,  1912, 
The  Pacific  Telephone  and  Telegraph  Company  had  started  an  investi- 
gation of  inductive  interference  between  the  lines  of  the  Coast  Counties 
Gas  and  Electric  Company  and  the  lines  of  the  telephone  company  in 
the  neighborhood  of  Morgan  Hill  in  Santa  Clara  County.  This  inves- 
tigation was  completed  by  the  Committee  and  its  results  have  been 
considered  in  connection  with  other  work  carried  out  by  the  Committee. 

In  January,  1913,  the  Committee  established  its  field  staff  at  Salinas, 
to  investigate  parallels  on  the  line  of  the  Sierra  and  San  Francisco 
Power  Company  north  of  Salinas  and  on  the  line  of  the  Coast  Valleys 
Gas  and  Electric  Company  south  of  Salinas,  both  of  these  power  lines 
being  parallel  with  the  lines  of  The  Pacific  Telephone  and  Telegraph 
Company,  the  Western  Union  Telegraph  Company  and  the  Southern 
Pacific  Company's  signalling  system.  The  investigation  at  Salinas  con- 
tinued from  January,  1913,  until  July,  1913. 

The  work  undertaken  at  Salinas  was  for  the  purpose  of  determining 
(1)  the  magnitude  and  characteristics  of  the  induction  produced  in  the 
communication  circuits,  the  factors  in  the  power  circuit  causing  this 
induction  and  the  quantitative  relationships  involved;  and  (2)  the  effect 
of  the  condition  of  the  neutral  (grounded  or  nongrounded)  of  the 
autotransformers  at  Salinas,  on  the  induction  in  the  communication 
circuits. 

Tests  were  made  of  the  induction  in  the  communication  circuits,  both 
north  and  south  of  Salinas,  under  operating  conditions  of  the  power 
circuits  and  with  the  neutral  at  Salinas  alternatively  grounded  and 
nongrounded.  Tests  were  also  made  with  special  methods  of  energizing 
the  power  circuit,  in  order  to  determine  the  relative  importance  of 
various  factors  in  causing  the  induction.  This  determination  was  also 
made  by  theoretical  methods,  by  computation  of  induction  based  upon 
the  dimensions  of  the  parallel  involved,  and  the  results  compared  with 
those  of  the  tests.  Instrument  transformer  equipment  was  investi- 
gated in  order  to  determine  the  errors  thereby  introduced  in  measure- 
ments of  the  power-circuit  voltages  and  currents. 

In  July,  1913,  the  field  headquarters  were  moved  to  Santa  Cruz. 
At  this  point  the  Committee  desired  to  test  the  relative  merits  of  various 
schemes  of  transpositions*  for  both  power  and  telephone  circuits,  and  to 
complete  the  investigation  begun  at  Morgan  Hill  on  the  system  of  the 
Coast  Counties  Gas  and  Electric  Company,  which  system  is  of  a  dif- 
ferent character  from  that  studied  at  Salinas.  A  mathematical  study 
of  transpositions  in  general,  and  particularly  of  those  for  the  parallel 
between  Santa  Cruz  and  Watsonville  was  completed. 

During  the  time  the  field  headquarters  of  the  Committee  were  at 
Santa  Cruz,  the  report  of  the  Committee  to  the  Railroad  Commission, 

*For  definition  of  "transposition"  see  page  32. 


FINAL  REPORT  ON  INDUCTIVE  INTERFERENCE  15 

dated  'July  7,  1914.  was  presented.  This  report  contained  ;m  account 
of  the  formation  of  the  Committee,  its  activities  and  the  results  accom- 
plished up  to  that  date,  and  also  included  such  recommend  jit  ions  for 
rulings  by  the  Railroad  Commission,  as  seemed  justified  to  the 
(  omiiiittee  at  that  time.  In  addition,  there  was  given  a  program  of 
future  work  designed  to  put  the  Committee  in  possession  of  informa- 
tion which  would  permit  making  the  recommended  ridings  more  definite 
and  complete. 

In  a  letter  of  acknowledgment  to  the  Committee  the  Railroad  Com- 
mission approved  the  program  of  future  work  which  was  laid  down 
in  the  report  and  authorized  the  continuance  of  the  Committee's 
investigations.  This  program  comprised  experimental  studies  both  of 
transpositions  and  of  residual*  voltages  and  currents  of  power  circuits. 
The  study  of  transpositions  included:  (1)  the  determination  of  the 
practical  effectiveness,  in  reducing  induction,  of  systems  of  power  and 
communication  circuit  transpositions  properly  co-ordinated  with  each 
other,  with  consideration  of  different  lengths  of  balanced  sections;  (2) 
the  influence  of  imperfect  electrical  balance  of  communication  circuits 
in  impairing  the  effectiveness  of  transposition  systems,  and  (3)  practical 
effectiveness  of  transpositions  in  a  power  circuit  isolated  from  ground  in 
balancing  the  voltages  between  the  several  conductors  and  ground,  with 
consideration  of  the  relative  efficiency  of  barrels**  of  different  lengths. 

The  study  of  residual  voltages  and  currents  included  an  experimental 
investigation  of  different  types  of  power  system  connections  and 
apparatus  with  respect  to  the  production  of  residual  voltages  and  cur- 
rents, of  means  to  be  employed  to  limit  their  magnitudes  and  the 
determination  of  the  minimum  values  which  will  produce  harmful 
inductive  interference. 

The  work  outlined  in  this  program  was  continued  at  Santa  Cruz  until 
November  24,  1914.  The  experimental  study  of  the  effectiveness  of 
transpositions  in  power  and  communication  circuits  undertaken  at  this 
point  could  not  be  carried  out,  due  to  lack  of  suitable  equipment.  An 
investigation  of  the  effect  of  various  transformer  connections  and  of 
the  magnetic  density  employed  in  transformer  iron  on  the  residual 
voltages  and  currents  introduced  in  grounded-neutral  networks  by  such 
transformers  was  begun.  In  addition,  from  Santa  Cruz  as  head- 
quarters, measurements  were  made  with  portable  apparatus  at  various 
points  on  the  systems  of  the  Coast  Counties  Gas  and  Electric  Company, 
the  Sierra  and  San  Francisco  Power  Company,  and  the  Pacific  Gas  and 
Electric  Company  in  order  to  study  their  characteristics  with  respect 
to  residual  voltages  and  currents. 

On  November  24,  1914,  the  field  headquarters  and  laboratory  of  the 
Committee  were  moved  to  San  Fernando.  This  location  offered  a 
number  of  advantages  for  experimental  work,  the  chief  one  being  the 
presence  of  an  unused  thirty-seven  mile  15,000-volt  line  of  the  Pacific 
Light  and  Power  Corporation  which  was  available  for  testing  at  all 
times.  A  telephone  circuit  carried  on  the  same  poles  with  the  15,000- 
volt  circuit  was  also  available,  constituting  a  parallel  for  experimental 
purposes.  In  addition,  transformers  were  loaned  by  the  Pacific  Light 
and  Power  Corporation  which,  with  other  transformers  already  pro- 

*For  definition  of  "residual"  see  page  33. 
**For  definition  of  "barrel"  see  page  32. 


16  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

vided  by  the  Sierra  and  San  Francisco  Power  Company,  gave  oppor- 
tunity for  carrying  out  transformer  studies. 

The  principal  experimental  work  undertaken  at  San  Fernando 
comprised  studies  of  the  factors  affecting  the  residual  voltages  and 
currents  of  power  circuits,  the  effectiveness  of  transpositions  in 
balancing  power  circuits  and  in  neutralizing  inductive  effects,  and  the 
magnitude  of  inductive  effects  in  short,  uniform,  nontransposed  sections 
of  parallel.  Concerning  the  residual  voltages  and  currents  of  power 
circuits  isolated  from  ground,  the  investigations  included  the  effects  of 
transpositions,  leakage,  accidental  grounds  and  frequency  of  alter- 
nations. Concerning  the  residuals  of  grounded-neiitral  circuits  the 
investigation  included  the  effects  of  magnetic  density  of  the  trans- 
formers and  of  various  connections  of  the  transformer  banks.  In 
preparing  for  the  latter  study  a  difficulty  was  encountered,  due  to  large 
double-frequency  residual  voltages  and  currents,  apparently  peculiar 
phenomena  previously  unrecorded  and  probably  of  rare  occurrence  in 
practice.  These  were  investigated  to  a  very  limited  extent,  for  the 
purpose  of  devising  means  to  overcome  them  so  that  the  programmed 
tests  might  be  carried  out. 

The  availability  of  both  an  idle  power  circuit  and  an  idle  telephone 
circuit,  the  conditions  of  which  could  be  varied  for  experimental  pur- 
poses, gave  an  excellent  opportunity  for  studying  induction  and  the 
effect  thereon  of  both  power  and  telephone  circuit  transpositions  and 
of  telephone  circuit  unbalances.  An  extensive  series  of  tests  was  made 
to  determine  the  ratios  of  induced  voltage  in  the  telephone  circuit  to 
inducing  voltage  or  current  in  the  power  circuit  under  different  con- 
ditions of  operation  of  the  power  circuit,  for  a  short,  uniform,  non- 
transposed  section  of  parallel.  These  ratios,  termed  coefficients  of 
induction,  were  also  obtained  independently  by  calculations  based  upon 
physical  dimensions  of  the  line.  The  results  of  the  two  independent 
determinations  were  compared  to  ascertain  the  practicability  of  obtain- 
ing coefficients  of  induction  for  other  cases  by  computations  thereby 
eliminating  the  necessity  for  tests.  Advantage  was  taken  of  the 
opportunity  at  San  Fernando  to  measure  the  residual  voltages  and 
currents  of  the  Pacific  Light  and  Power  Corporation's  15,000- volt 
system  to  supplement  the  similar  measurements  previously  made  on 
other  systems. 

On  June  17,  1915,  the  headquarters  of  the  field  staff  were  moved  to 
San  Francisco,  where  the  work  of  analyzing  the  San  Fernando  data 
was  completed.  At  San  Fernando  the  Committee's  energies  were 
largely  centered  on  completing  the  experimental  work,  before  the  power 
line  was  required  for  service. 

It  was  endeavored  to  make  the  analyses  and  reports  as  thorough  and 
complete  as  possible.  For  each  of  the  subjects  experimentally  investi- 
gated at  San  Fernando,  theoretical  studies  were  undertaken  at  San 
F'rancisco,  which  in  some  cases  were  much  extended  in  scope  over  that  of 
the  corresponding  experimental  work.  Where  possible,  the  experi- 
mental and  theoretical  results  were  compared.  The  effects  of  circuit 
configuration,  or  arrangement  and  relative  location  of  conductors, 
transpositions  and  frequency  on  the  residual  voltages  and  currents  due 


FINAL    RKI'OUT    ON    INDUCTIVE    INTERFERENCE  17 

to  the  line  unbalance  of  power  circuits  isolated  from  ground,  and  1lic 
effects  of  accidental  grounds,  were  investigated  from  a  theoretical 
standpoint.  A  study  was  made  of  the  relation  of  magnetic  density  of 
the  transformer  iron  and  of  transformer  connections  to  the  residual 
voltages  and  currents  of  triple  frequencies  thereby  introduced  into 
connected  circuits.  A  report  was  prepared  giving  formulas  for  th<- 
computation  of  coefficients  of  induction  in  communication  circuits 
paralleled  by  power  circuits,  including  an  explanation  of  the  derivation 
of  the  formulas  and  convenient  forms  which  had  been  developed  for 
systematically  carrying  out  such  computations. 

To  determine  the  effect  of  configuration  and  relative  position  of 
power  and  communication  circuits  on  the  induction  in  the  latter,  an 
extensive  series  of  computations  based  upon  the  dimensions  of  assumed 
cases  of  parallelism,  was  carried  out.  The  results  of  this  study  comprise 
214  curve  sheets,  containing  over  3,000  curves,  by  the  aid  of  which  the 
values  of  the  coefficients  of  induction  for  those  cases  of  parallelism 
Avhich  occur  most  commonly  may  be  determined. 

Among  the  important  reports  prepared  at  San  Francisco  is  one 
reviewing  previous  work  and  presenting  new  data  on  the  subject  of 
co-ordinating  power-circuit  and  telephone-circuit  transpositions  as  a 
means  of  reducing  interference.  A  new  telephone  transposition  sys- 
tem developed  by  the  American  Telephone  and  Telegraph  Company, 
largely  in  response  to  the  need  for  a  system  of  telephone  transpositions 
having  increased  flexibility  in  respect  to  co-ordination  with  power- 
circuit  transpositions,  is  described  and  its  use  illustrated.  As  examples 
of  co-ordinated  transposition  systems,  plans  are  presented  for  all  the 
parallels  which  have  been  experimentally  investigated  by  the 
Committee. 

Apparatus  suitable  for  use  in  the  experimental  work  of  the  Com- 
mittee was  not  easily  obtainable  and  in  many  instances  it  was  necessary 
to  design  and  develop  special  apparatus  for  certain  of  the  tests.  In 
cases  where  apparatus  was  not  available  for  measuring  desired 
quantities  directly,  it  was  necessary  to  develop  methods  of  measurement 
whereby  they  might  be  obtained  indirectly. 

In  deciding  from  time  to  time  upon  its  program  for  future  work,  the 
Committee  has  found  it  necessary  to  formulate  and  consider  in  detail 
many  plans  of  experimentation  which  have  never  been  carried  out.  It 
has  not  always  been  easy  to  decide  upon  the  best  location  for  carrying 
on  a  particular  investigation  when  each  of  the  several  different  possible 
locations  possessed  certain  advantages.  To  decide  between  them  or  to 
choose  between  different  programs  of  work  has  meant  that  the  several 
plans  under  consideration  had  to  be  worked  up  in  considerable  detail 
before  the  preponderance  of  advantage  in  favor  of  some  one  procedure 
could  be  established.  In  several  cases  plans  for  work  regarded  as 
particularly  desirable  had  to  be  given  up  because  they  were  found  to 
be  too  laborious,  or  for  other  reasons  were  not  feasible. 

In  the  course  of  the  investigation  seventy-one  Technical  Reports  have 
been  prepared,  which  describe  in  detail  the  various  features  of  the  work, 
the  method  and  apparatus  employed  and  the  results  accomplished. 
These  reports,  some  of  which  are  recommended  for  publication,  are 
listed  in  Appendix  II. 


18  FINAL    REPORT    ON   INDUCTIVE    INTERFERENCE 

At  the  request  of  the  Committee,  laboratory  investigations  were  made 
in  New  York  by  the  American  Telephone  and  Telegraph  Company,  the 
Postal  Telegraph-Cable  Company  and  the  Western  Union  Telegraph 
Company  to  determine  the  detrimental  effects  of  extraneously  induced 
currents  on  the  operation  of  telephone  and  telegraph  circuits.  Reports 
of  the  results  of  these  investigations  were  submitted  to  the  Committee. 
These  are  also  listed  in  Appendix  II. 

At  various  times  during  the  course  of  its  work,  the  Committee  has 
contributed  discussions  before  the  American  Institute  of  Electrical 
Engineers.  The  Committee's  report  of  July  7,  1914,  was  presented  at 
the  Spokane  Convention  of  the  Institute  in  September,  1914,  and  later 
at  meetings  of  the  San  Francisco  and  of  the  Los  Angeles  sections  of  the 
Institute.  On  each  of  these  occasions  considerable  discussion  was 
brought  forth.  In  June,  1915,  at  the  Deer  Park  Convention  in  connec- 
tion with  papers  presented  on  the  subject  of  irregular  power -circuit 
wave-forms,  the  Committee  submitted  a  discussion  from  the  standpoint 
of  inductive  interference.  In  September,  1915,  at  the  Panama-Pacific 
Convention  the  Committee  submitted  a  discussion  in  which  the  progress 
of  the  work  from  July,  1914,  to  September,  1915,  was  described.  In 
September,  1916,  at  the  convention  of  the  Institute  held  in  Seattle, 
the  Committee  submitted  a  discussion  of  a  paper  presented  on  the 
subject  of  irregular  wave-forms. 

Finances. 

The  funds  required  for  carrying  on  the  work  of  the  Committee  were 
contributed  by  various  telephone,  power  and  telegraph  companies. 
Such  contributions  were  made  at  the  start  of  the  investigation  and 
immediately  after  the  report  rendered  to  the  Commission  on  July  7, 
1914.  Further  support  was  given  in  the  furnishing,  by  the  railroad 
companies,  of  free  transportation  to  the  Committee  members  and 
employees  while  on  Committee  business,  and  of  the  Committee's  equip- 
ment ;  by  the  telephone  companies  of  the  services  of  their  engineers  on 
the  work  of  the  field  staff;  and  by  the  Railroad  Commission  of  the 
stenographer  and  stationery  supplies.  The  time  which  the  Committee 
members  devoted  to  the  work  was  without  cost  to  the  Committee. 

It  is  estimated  that  the  total  cost  of  the  investigation  is  more  than 
$100,000. 


FINAL    KEPOKT    ON    INDTCTIVI-;    I NTKKFKUKNCE  19 

PART  TWO. 

EXPLANATION  OF  PROBLEM  AND  SUMMARY  OF  RESULTS. 
Nature  of  subject. 

The  object  sought  herein  is  to  describe  \vluM  inductive  interference  is. 
using  as  far  as  practicable  nontechnical  terms,  for  the  benefit  of  those 
not  familiar  with  electrical  theory. 

The  transmission  of  power  electrically  by  wire  circuits  in  either 
large  or  small  quantities  requires  a  current  of  electricity.  Also,  to 
make  electricity  flow,  there  must  be  in  the  circuit  a  voltage  or,  in  other 
words,  electric  pressure,  as  all  circuits  offer  more  or  lass  resistance  or 
impedance  to  an  electric  current.  If  the  voltage  is  produced  directly 
by  a  battery  it  forces  the  electric  current  around  the  circuit  in  one 
direction  only.  Such  current  is  called  direct  or  continuous.  Con- 
tinuous voltage  and  current  may  also  be  produced  by  an  electric 
generator,  and  this  is  the  common  practice  for  street  railways, 
but  on  most  other  power  lines  the  generators  produce  an  alternating 
voltage, — that  is  a  voltage  which  during  each  short  interval  of  time 
known  as  a  period  (usually  not  longer  than  one  twenty-fifth  of  a 
second)  varies  in  value  from  zero  up  to  a  maximum,  then  diminishes  to 
zero,  increases  to  a  maximum  in  the  opposite  direction,  and  then 
diminishes  again  to  zero,  repeating  this  cycle  of  variations  through  suc- 
ceeding equal  periods.  Thus,  the  voltage  and  the  corresponding  cur- 
rent change  in  direction  or  alternate  twice  each  period.  The  number 
of  periods  or  cycles  per  second  is  called  the  frequency. 

The  voltage  associated  with  any  electric  circuit  is  accompanied  by  an 
electric  field  of  force,  or  condition  of  stress,  in  the  surrounding  space, 
whose  intensity  is  proportional  to  the  voltage.  At  the  same  time  the 
corresponding  electric  current  is  accompanied  by  a  magnetic  field  of 
force  which  occupies  the  same  surrounding  space  and  whose  intensity 
is  proportional  to  the  current.  Thus  any  changes  in  the  magnitude  or 
direction  of  the  voltage  and  current,  such  as  the  alternations  described 
above,  are  accompanied  by  corresponding  changes  in  their  fields.  The 
intensity  of  these  fields  of  force,  in  general,  diminishes  very  rapidly 
with  increasing  distance  from  the  circuit. 

Conversely,  any  other  circuit  within  these  fields  of  force  will  have 
voltages  and  currents  set  up  or  ''induced"  in  it,  when  changes  occur 
in  the  fields,  that  is,  when  the  voltage  or  current  of  the  first  circuit 
changes.  Power  circuits  of  the  alternating-current  type,  most  com- 
monly employed  in  power  transmission,  having  their  voltages  and 
currents  continually  varying,  will  continually  induce  voltages  and  cur- 
rents in  a  neighboring  communication  circuit.  These  induced  voltages 
and  currents  are  evidence  of  the  absorption  of  energy  from  the  fields. 
Thus  one  circuit  influences  another  by  the  transfer  of  energy  from  the 
one  to  the  other,  without  any  contact  between  the  wires  of  the  two  cir- 
cuits. This  phenomenon,  termed  "induction,"  has  long  been  known, 
and  has  many  useful  applications  in  electrical  engineering. 

To  transmit  signals  over  a  communication  circuit  it  is  necessary  thai 
the  power  used,  and  thus  the  voltage  and  current,  vary  from  instant  to 
instant.  In  telephone  circuits  this  variation  is  extremely  complex,  the 


20  FINAL    REPORT   ON   INDUCTIVE   INTERFERENCE 

current  which  reproduces  the  human  voice  in  a  distant  telephone  con- 
sisting of  a  number  of  component  simple  currents  varying  in  frequency 
from  about  100  to  4,000  cycles  per  second.  For  telegraph  circuits  the 
variation  is  much  less  complex  and  the  frequencies  of  the  important 
components  of  the  voltage  and  current  are  less  than  300  cycles  per 
second.  In  both  cases  the  signalling  impulses  are  sent  and  received  by 
delicate  mechanisms,  and  the  amounts  of  power  required  are  exceed- 
ingly small,  particularly  for  telephone  circuits.  When  communication 
circuits  are  in  the  field  of  influence  of  a  power  circuit  the  rate  at  which 
energy  is  transferred  to  them  by  induction  may  be  comparable  with, 
or  even  larger  than,  the  power  required  for  their  operation,  although 
entirely  inappreciable  compared  to  that  of  the  power  circuit.  For 
example,  the  power  required  to  operate  a  small  incandescent  lamp  is 
sufficient,  if  directly  applied,  to  cause  a  loud  noise  in  several  million 
telephone  receivers. 

For  power  circuits  of  the  type  most  commonly  used  in  California, 
the  frequency  is  either  50  or  60  cycles  per  second.  This  is  the  funda- 
mental frequency  of  the  voltage  and  current,  representing  useful  power, 
but  there  are  also  present  in  power  circuits  other  voltages  and  currents, 
usually  of  relatively  small  magnitude,  of  various  higher  frequencies  up 
to  several  hundred  cycles  per  second.  These  higher  frequencies  or 
harmonics  of  the  fundamental  frequency,  are  the  chief  cruise  of  inter- 
ference to  telephone  circuits,  since  they  are  of  the  frequencies  of  the 
sound-waves  of  the  human  voice,  at  which  the  telephone  is  most 
sensitive.  On  the  other  hand,  the  chief  interference  with  telegraph 
circuits  is  caused  by  the  fundamental  or  useful  frequency  of  the  power 
circuits,  which  most  nearly  corresponds  to  the  frequency  of  the  tele- 
graphic impulses. 

The  disturbances  thus  caused  in  telephone  circuits  manifest  them- 
selves as  humming  noises  which  impair  the  intelligibility  of  conversa- 
tion and  cause  annoyance.  In  telegraph  circuits,  chattering  of  the 
relays  is  caused,  the  intelligibility  of  signals  is  impaired,  and  the  speed 
and  ease  of  transmission  are  reduced. 

Under  abnormal  conditions  the  inductive  disturbance  due  to  a  power 
circuit  may  be  very  greatly  increased.  When  sudden  changes  take 
place  in  the  conditions  of  the  power  circuit  such  as  those  caused  by 
energizing  or  de-energizing  the  circuit,  or  when  a  wire  breaks  and  falls  to 
ground,  relatively  large  amounts  of  energy  may  be  suddenly  introduced 
into  the  communication  circuits.  These  momentary  impulses  may  be 
sufficient  to  constitute  a  physical  hazard,  to  operate  protective  devices  or 
to  cause  severe  acoustic  shocks  to  telephone  operators  or  users. 

Briefly,  then,  inductive  interference  may  be  defined  as  the  impair- 
ment of  the  serviceability  of  communication  circuits  resulting  from  the 
transference  of  energy  into  them,  through  intervening  space,  from 
near-by  power  circuits.  The  study  of  inductive  interference  deals 
with  the  factors  affecting  the  magnitude  and  character  of  the  induction 
and  their  relationships,  the  attendant  detrimental  effects  on  communi- 
cation circuits  and  the  means  to  be  employed  in  overcoming  or 
mitigating  such  interference. 


FINAT,    RKI'OKT    <>N     I  MMTTIVK    1  NTlvKFKRENCE  2l 

Summary  of  facts  established. 

It  serins  desirable  to  summarize  briefly  Hie  principal  technical  facts 
regarding  inductive  interference,  which  may  now  be  considered  as 
established.  Only  the  most  important  points  are  mentioned  here,  a 
detailed  technical  discussion  being  given  in  the  Technical  Reports.* 

1.  Primary  Cause. 

As  previously  shown  in  discussing  the  "Nature  of  Subject,"  the 
primary  cause  of  inductive  interference  is  the  presence,  about  the 
power  circuits,  of  fields  of  influence  which  vary  in  intensity  from 
instant  to  "instant,  usually  in  periodic  or  cyclic  fashion.  Communica- 
tion circuits  in  regions  where  these  fields  are  of  appreciable  strength 
absorb  energy  therefrom,  by  "induction."  When  the  rate  at  which 
the  energy  is  thus  absorbed  is  of  the  same  order  of  magnitude  as  the 
power  required  for  the  transmission  of  signals,  the  impulses  received 
at  the  terminals  of  the  communication  circuit  are  distorted  and  the 
serviceability  of  the  circuit  is  impaired. 

2.  Interference  to  Telephone  Circuits — Harmonics. 

Under  normal  operating  conditions  of  the  disturbing  power  circuits, 
interference  to  telephone  circuits,  manifested  by  a  humming  noise  from 
the  telephone  receivers,  is  due  almost  entirely  to  the  higher  harmonics 
of  the  power-circuit  voltages  and  currents;  for  the  reason  that  such 
harmonics  cover  a  considerable  portion  of  the  range  of  frequencies  of 
human  speech,  at  which  telephone  apparatus  is  most  sensitive. 
Except  when  the  interference  is  very  slight  or  very  severe,  the  detri- 
mental effect  of  extraneous  current  in  a  telephone  receiver  increases 
approximately  in  direct  proportion  to  the  magnitude  of  the  current. 
Increasing  the  frequency  causes  a  very  rapid  increase  in  the  detri- 
mental effect  (roughly  as  the  square  of  the  frequency)  up  to  about  800 
periods  per  second,  beyond  which  there  is  a  gradual  decrease.  When 
several  frequencies  are  present  in  the  extraneous  current,  the  resultant 
detrimental  effect  is  considered  roughly  proportional  to  the  square  root 
of  the  sum  of  the  squares  of  the  separate  effects  of  the  several  single- 
frequency  components,  though  this  relation  has  not  been  definitely 
established. 

The  higher  harmonics,  which  are  irregularities  of  the  voltage  and 
current  waves  of  power  circuits,  usually  result  from: 

(a)  design  and  construction  of  generators  and  motors,  whereby 
pure  sine  wave  shapes  are  only  approximated; 

(b)  the  use  of  iron  in  transformers  under  conditions  approach- 
ing magnetic  saturation,  thereby  causing  distortion  of  the  current 
and  voltage  waves j 

((')  the  presence  of  electric  arcs  in  the  circuit,  as  in  some  street- 
lighting  systems. 

The  higher  harmonics  which  commonly  occur  in  alternating-current 
systems  are  odd  integral  multiples  of  the  fundamental  frequency. 
They  are  of  sufficient  magnitude  to  be  of  importance,  often  as  high  as 
nineteen  times  the  fundamental  frequency,  and  have  been  observed  as 

*Listed  in  Appendix  II. 

4—39828 


22  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

high  as  the  35th  order.  High  frequency  voltages  and  currents  also  occur 
in  direct-current  systems.  Harmonics  (other  than  the  fundamental 
or  first  harmonic)  are  not  essential  to  the  functioning  of  power  systems 
and  may  be  sources  of  trouble  therein. 

Induction  of  the  fundamental  frequency  of  power  circuits  (below  100 
cycles  per  second)  is  the  cause  of  very  little  interference  to  telephone 
circuits,  except  when  its  magnitude  is  sufficient  to  constitute  a  physical 
hazard,  or  to  operate  grounded  signalling  devices,  as  both  the  human 
ear  and  telephone  apparatus  are  much  less  sensitive  to  these  relatively 
low  frequencies. 

3.  Interference  to  Telegraph  Circuits. 

Under  normal  operating  conditions  of  the  disturbing  power  circuits, 
interference  to  telegraph  circuits,  manifested  by  the  reduction  in  clear- 
ness and  maximum  speed  of  signalling,  is  due  to  induced  currents  of 
fundamental  frequency  and,  to  a  limited  extent,  frequencies  of  the 
lower  harmonics  (chiefly  the  third). 

Telegraph  receiving  instruments  are  readily  responsive  to  these 
frequencies,  because  they  approach  the  normal  operating  frequencies 
of  telegraph  transmission.  Telegraph  instruments  are  not  sensitive  to 
the  higher  harmonics.  Other  signal  circuits  (telephone  circuits 
excluded),  in  general,  resemble  telegraph  circuits  in  being  most  affected 
by  induced  currents  of  fundamental  frequency. 

4.  Balanced  and  Residual  Components. 

In  analyzing  inductive  effects,  it  is  convenient  to  divide  the  power- 
circuit  voltages  and  currents  into  two  general  classes:  (1)  "balanced," 
with  respect  to  the  earth  as  a  neutral  conductor  or  point  of  reference, 
and,  (2)  "residual,"  completely  unbalanced  with  respect  to  the  earth, 
i.e.,  employing  the  metallic  power-circuit  conductors,  as  a  group,  for  one 
"side"  and  the  earth  as  the  other  side  of  their  circuit. 

'  *  Balanced ' '  current  components  in  the  several  conductors  of  a  power 
circuit  are  such  that  at  every  instant  their  algebraic  sum  is  zero.  The 
algebraic  sum  of  the  total  currents  in  the  several  conductors  of  a  power 
circuit  at  any  instant  is  the  "residual"  current.  Similarly,  the 
"balanced"  voltages  of  the  several  conductors  are  such  that  their  alge- 
braic sum  is  zero  at  every  instant,  while  the  algebraic  sum  of  the 
total  voltages  to  ground  at  any  instant  is  the  "residual"  voltage. 

As  an  example,  a  trolle}^  circuit,  consisting  of  an  overhead  trolley 
wire  and  "return"  through  rails  and  earth,  is  completely  unbalanced 
with  respect  to  the  earth,  its  total  voltage  and  current  being  residual. 
On  the  other  hand,  a  two-wire  circuit  having  no  metallic  connection  to 
earth  and  its  two  sides  symmetrical  with  respect  to  the  earth's  surface 
and  not  in  close  proximity  to  other  circuits  or  objects,  would  have  no 
residuals,  the  voltages  to  earth  of  the  sides  of  the  circuit  being  equal  and 
opposite  and  the  currents  wholly  confined  to  the  metallic  conductors  and 
therefore  equal  and  opposite,  i.e.,  in  both  cases  balanced. 

This  classification  of  the  voltages  and  currents  is  of  basic  importance, 
since  there  is  no  generally  applicable  relation  between  balanced  and 
residual  components  or  their  inductive  effects,  and  furthermore  since 
the  remedies  for  induction  from  balanced  and  residual  voltages  or  cur- 
rents are  often  fundamentally  different. 


FINAL   REPORT    ON    INDUCTIVE   INTERFERENCE  23 

The  circuHs  commonly  employed  in  power  transmission  jind 
distribution  ordinarily  have  both  classes  of  voltages  and  currents  in 
sufficient  magnitude  to  require  attention.  With  exceptions,  such  as 
trolley  circuits  above  mentioned,  the  balanced  components  of  funda- 
mental frequency  are  the  useful  energy-transferring  agents,  while  the 
residuals  are  the  result  of  incidental  differences  between  ideal  design 
and  construction  of  line  and  apparatus,  giving  perfect  balance,  and 
design  and  construction  which  approach  this  condition  sufficiently 
for  commercial  operation,  disregarding  inductive  effects. 

Both  balanced  and  residual  voltages  and  currents  contain  harmonics, 
but  the  general  tendency  is  that  the  residuals  contain  greater  percent- 
ages of  harmonics  than  do  the  balanced  components.  Besides,  under 
some  conditions  (discussed  in  5  below),  a  series  of  harmonics,  odd 
multiples  of  three  times  the  fundamental  frequency,  appear  as  residuals, 
but  not  in  the  balanced  components. 

Inductive  effects  from  residuals  are  usually  of  greater  intensity  than 
those  from  balanced  voltages  or  currents  of  equal  magnitude.  The 
ratio  of  effects  from  these  two  sources  is  exceedingly  variable,  ranging 
from  about  two  to  several  thousand.  The  relatively  greater  induction- 
producing  power  of  residuals  is  due  to  the  fact  that  the  residual  com- 
ponents associated* with  the  several  conductors  are  all  "in  phase"  and 
their  inductive  effects  therefore  cumulative,  whereas  the  several 
balanced  components  are  "out  of  phase"  (by  120  degrees  in  a  three- 
phase  system)  and  hence  their  resultant  induction  is  a  differential 
effect,  i.e.,  the  inductive  effects  due  to  the  balanced  components  partially 
neutralize  one  another. 

5.  Causes  and  Remedies  for  Residuals. 

Unbalances  or  inequalities  among  the  admittances  to  ground  of  the 
several  conductors  of  a  power  circuit  cause  residuals  of  the  frequencies 
present  in  the  voltages  between  conductors.*  In  a  system  without 
metallic  connection  to  earth  a  residual  voltage  is  produced.  With  a 
grounded-neutral  system  a  residual  current  is  produced  and  the  residual 
voltage  due  to  unbalanced  line  admittances  is  greatly  reduced.  Unbal- 
anced admittances  are  caused  by :  ( 1 )  differences  of  position  of  the 
conductors  with  respect  to  ground  and  to  one  another,  being  a  function 
of  the  configuration,  height  above  ground,  location  of  ground-wires  and 
other  neighboring  objects,  and  to  a  small  extent,  of  size  of  conductors ; 
and  (2)  differences  in  insulation  resistance,  as  may  be  due  to  defective 
insulators.  Transposing  the  conductors,  which  tends  to  equalize  their 
relations  to  ground  and  to  one  another,  is  an  effective  remedy  for 
unbalanced  capacitance.  Such  transpositions  must  be  located  with 
proper  regard  to  changes  in  configuration,  and  at  short  enough 
distances  from  each  other  so  that  there  is  no  material  difference  in  the 
electrical  conditions  at  two  such  points  at  any  given  instant.  Of 
commonly  occurring  configurations  the  equilateral  triangular  is  most 
nearly  balanced,  hence  causes  the  least  residuals  due  to  unbalanced 
capacitances,  while  the  plane  configurations,  especially  the  unsym- 
metrical  horizontal,  are  the  worst  in  this  respect.  The  remedy  for 

*It  is  to  be  noted  that  the  unbalances  here  referred  to  are  not  unbalances  such  as 
those  due  to  single-phase  loads  between  line  conductors. 


24  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

unbalanced  insulation  resistance  lies  in  careful  maintenance ;  for  a  well 
constructed  and  maintained  system  this  is  usually  not  an  important 
source  of  residuals. 

In  a  power  system  having  loads  connected  between  the  several  con- 
ductors and  ground  (as  in  a  star-connected  system  with  grounded 
neutrals),  differences  among  the  loads  of  the  several  phases  may  cause 
residual  voltages  and  currents,  due  to  part  of  the  load  being  supplied 
through  a  circuit  consisting  of  conductors  with  ground  "return." 
Inequalities  of  ratios  or  impedances  among  the  transformers  of  a  bank 
also  cause  residuals  in  such  a  circuit.  The  evident  remedy  is  careful 
equalization  of  loads,  and  the  use  of  like  transformers.  Removal  of 
the  ground  path  for  unbalanced  load  currents,  allowing  only  one 
neutral  ground,  is  the  most  effective  and  reliable  remedy  for  this  source 
of  residual  current. 

When  a  transformer  bank  in  a  three-phase  system  is  connected  in 
star  with  neutral  grounded,  harmonics  of  three  times  the  fundamental 
frequency,  and  odd  multiples  thereof,  appear  as  residuals,  on  the 
grounded -neutral  side.  This  is  because  of  the  variation  of  the  per- 
meability of  the  transformer  iron  with  varying  magnetic  density, 
causing  harmonics  in  transformer  exciting  currents  or  in  their  induced 
voltages.  As  the  triple-harmonic  components  are  "in  phase"  in  the 
three  transformers,  triple-harmonic  residual  voltages  and  currents  are 
produced  if  the  neutral  is  grounded.  Delta-connected  windings  on  such 
a  transformer  bank  provide  a  shunt  path  for  these  triple-harmonic  com- 
ponents of  the  exciting  current,  and  greatly  lessen  the  residuals  which 
might  otherwise  be  caused  on  the  grounded  neutral  side.  Since  the 
magnitude  of  these  residuals  decreases  very  rapidly  as  the  maximum 
magnetic  density  is  reduced,  lowering  the  voltage  impressed  per  turn 
of  the  transformer  winding,  or  substituting  transformers  of  lower  mag- 
netic density,  is  a  very  effective  remedy.  Isolating  the  neutral  of  a 
transformer  bank  eliminates  it  as  a  source  of  triple-harmonic  residuals. 

Generators  with  star-connected  armature  windings  may  cause 
residuals  due  to:  (1)  inequalities  among  the  voltages  induced  in  the 
several  windings;  (2)  departure  from  ideal  phase  differences,  120 
degrees  for  a  three-phase  generator;  (3)  triple-harmonic  voltages  of  the 
three  windings  being  in  phase,  between  neutral  and  line  terminals. 
When  a  generator  is  connected  to  the  line  either  directly  or  through 
auto-transformers,  residuals  are  thus  caused  only  if  the  generator 
neutral  is  grounded.  When  connected  to  the  line  through  transformers 
residuals  will  result  from  these  causes  if  the  transformer  bank  is  star- 
star  connected  with  line-side  neutral  grounded  and  station-side  neutral 
connected  to  generator  neutral.  The  remedies  are:  (1)  careful  design 
and  construction,  (2)  avoidance  of  grounded  neutral  or  transformer 
connections  permitting  transformation  of  generator  residuals  to  line 
(as  by  the  use  of  a  delta  connection  on  the  generator  side  of  the 
transformers). 

The  grounding  of  transformers,  transformer  banks  or  generators  at 
unsymmetrical  points  of  their  windings  unbalances  the  electrically  con- 
nected circuit  and  thereby  causes  a  residual  voltage  and  current.  The 
remedy  is  obvious. 


I'M  N  Ah    RKI'OKT    o.\     I.XWCTIVE    INTUKKKKKNCE  25 

6.  Factors  Affecting  Intensity  ami  Mat/nil  mlc  of  Induction . 

(a)  Dimensional  Factors.  In  general,  as  the  horizontal  separation  <>!' 
power  and  communication  lines  is  increased,  the  induction  decreases  at 
a  rate  varying,  roughly,  from  direct  proportionality  to  about  the  third 
power  of  the  separation.  That  is,  doubling  the  separation  reduces  the 
induction  from  unity  to  some  value  between  one-eighth  or  less  and 
one-half.  The  rate  of  decrease  is  less  for  magnetic  induction  than  for 
electric  induction  and  less  for  induction  in  grounded  circuits  than  for 
induction  in  metallic  circuits.  When  the  disturbed  and  disturbing  cir- 
cuits are  very  close  together,  the  rate  of  decrease  may  be  less  than 
stated  above  and  in  some  instances,  notably  with  the  vertical- 
configuration  power  circuit,  there  may  be  an  increase  of  induction  at 
first,  as  the  separation  increases. 

Other  things  being  equal,  the  magnitude  of  the  induction  increases 
nearly  in  direct  proportion  to  the  length  of  parallel. 

The  configuration  of  a  power  circuit  has  a  large  influence  on  the 
intensity  of  the  induction  from  balanced  voltages  and  currents,  but  a 
very  small  influence  on  induction  from  residuals.  No  one  configuration 
commonly  employed  can  be  selected  as  universally  superior,  since  the 
one  giving  the  least  inductive  effect  depends  on  the  spacing  of  the  con- 
ductors and  the  relative  position  of  the  two  classes  of  circuits,  also 
upon  the  type  of  induction,  electric  or  magnetic,  which  preponderates. 

Induction  from  balanced  components  increases  nearly  in  direct  pro- 
portion to  the  spacing  of  power  conductors,  but  induction  from 
residuals,  particularly  residual  current,  is  only  slightly  affected  by  the 
conductor  spacing. 

The  intensity  of  the  direct  induction  in  metallic  communication  cir- 
cuits depends  largely  upon  the  arrangement  of  the  conductors  and 
increases  in  direct  proportion  to  their  spacing  (for  two-conductor 
circuits  in  a  given  plane)  ;  but  the  inductive  effects  on  the  conductors 
as  a  group,  with  reference  to  the  earth  as  a  neutral  conductor,  are  only 
slightly  affected  by  the  spacing  or  arrangement. 

(b)  Electrical  Factors.  The  induced  current  in  a  communication 
circuit  increases  in  direct  proportion  to  the  magnitude  of  the  voltage  or 
current  in  the  power  circuit  which  causes  it,  and  approximately  in  pro- 
portion to  the  frequency  of  the  inducing  voltage  or  current. 

As  stated  above,  residual  voltages  or  currents  produce  much  more 
intense  inductive  effects  than  balanced  voltages  or  currents  of  the  same 
magnitude. 

The  magnitude  of  the  induced  current  is  considerably  affected  by 
the  amount  and  character  of  line  and  of  terminal  apparatus  between 
the  parallel  or  source  of  disturbance  and  the  receiving  instrument. 
The  primary  effect  of  such  sections  of  unexposed  line  and  apparatus  is 
to  diminish  the  received  current. 

Several  communication  conductors  on  one  line  tend  to  shield  one 
another.  It  is  generally  assumed  that  ground-wires,  as  commonly 
employed  for  lightning  protection  on  power  lines,  are  shielding  agents. 
This  is  true  with  respect  to  inductive  effects  from  residual  voltages  and 
currents,  but  such  ground-wires  may  increase  the  intensity  of  the 
induction  from  balanced  voltages  and  currents,  by  distortion  of  the 
electric  and  magnetic  fields  about  the  power  circuit. 


26  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

Using  the  well-known  laws  of  electricity  and  magnetism,  it  is  pos- 
sible to  determine  by  computations  the  effect  of  these  various  factors, 
both  dimensional  and  electrical,  in  simple  practical  cases.  Even  with 
the  simplifying  assumptions  allowable,  the  work  is  usually  tedious.  In 
complex  cases  as  when  the  simultaneous  action  of  all  factors  is  to  be 
considered,  quantitative  results  are  best  obtained  by  experimental 
means. 

7.  Transpositions. 

One  of  the  most  valuable  means  of  overcoming  inductive  interference 
under  normal  operating  conditions  of  power  circuits  is  to  transpose  the 
conductors  of  each  circuit,  so  as  to  equalize  their  relations  to  all  other 
circuits  and  to  earth. 

Transpositions  in  a  power  circuit  tend:  (1)  to  equalize  the 
capacitances  of  its  conductors  to  ground,  thereby  removing  a  source  of 
residuals,  and  (2)  to  cause  the  inductive  effects  from  the  balanced 
voltages  and  currents  to  neutralize  one  another  in  neighboring  lengths 
of  a  parallel  communication  line.  Transposition  of  a  power  circuit  does 
not  reduce  induction  from  residuals,  except  as  it  may  do  so  indirectly  by 
a  reduction  in  the  magnitude  of  the  residuals  as  just  noted. 

Transpositions  in  a  communication  circuit  tend:  (1)  to  equalize  the 
capacitances  of  its  conductors  to  ground;  (2)  to  lessen  the  induction 
among  the  several  communication  circuits  of  a  line  (known  as  "cross- 
talk" on  telephone  circuits)  ;  and  (3)  to  equalize  the  inductive  effects  on 
the  two  sides  of  the  circuits,  due  to  near-by  power  circuits.  Such  trans- 
positions do  not  protect  the  circuit  against  voltages  induced  between 
the  circuit  as  a  whole  and  ground  or  along  the  conductors  as  a  group. 

In  order  that  transpositions  shall  be  most  effective  they  must  be 
carefully  located,  within  sections  where  the  intensity  of  the  inductive 
effects  is  uniform,  with  respect  to  points  where  the  induction  changes, 
called  points  of  discontinuity.  The  transpositions  in  each  class  of  line 
must  also  be  located  with  regard  to  the  transpositions  of  the  other  class 
of  line,  i.e.,  the  transpositions  in  the  power  and  communication  lines 
must  be  co-ordinated. 

On  account  of  the  finite  (though  very  short)  time  required  for 
electric  waves  to  travel  along  the  conductors,  the  electrical  conditions 
at  a  given  instant  will  be  different  at  different  points  along  the  lines, 
being  practically  opposite  at  points  one-half  wave  length  apart;  hence 
transpositions  laid  out  on  a  basis  of  uniform  conditions  do  not  produce 
perfectly  neutralizing  and  equalizing  effects  in  adjacent  sections  of  a 
parallel.  To  be  effective,  therefore,  the  nominally  balanced  lengths  of 
a  transposition  scheme  should  be  very  short  as  compared  to  a  wave 
length  at  the  frequencies  of  induction  to  be  considered  and  guarded 
against.  The  impairment  of  balance  due  to  this  effect  varies  approxi- 
mately as  the  square  of  the  length  of  nominally  balanced  section  and 
directly  as  the  frequency  of  the  induction.  It  is  usually  advantageous 
to  omit  transpositions  at  the  junction  points  of  successive  balanced  sec- 
tions, or  barrels  of  the  power  circuit,  as  this  lessens  the  impairment  of 
balance  just  mentioned. 

The  length  of  parallel  within  which  a  nominal  balance  should  be 
obtained  in  a  scheme  of  transpositions  designed  to  adequately  reduce 


FINAL    KKI'ORT   ON   INDUCTIVE   INTERFERENCE  27 

interference,  is  usually  determined  by  the  points  of  discontinuity,  the 
lengths  of  sections  thus  required  adequately  meeting  the  requirement 
above 'mentioned  of  securing  balance  within  a  small  fraction  of  a  wave 
length.  In  long  uniform  parallels  involving  telephone  circuits,  bal- 
anced sections  with  barrels  in  the  power  circuits  three  miles  in  length 
are  usually  adequate.  For  such  parallels  involving  telegraph  circuits 
longer  barrels  are  permissible  as  only  the  wave  length  of  fundamental 
frequency  need  be  considered. 

Though  transpositions  afford  a  very  practical  and  effective  means  of 
mitigation  for  some  inductive  disturbances,  they  cannot  be  considered 
as  a  complete  remedy  for  interference  even  under  normal  operating 
conditions. 

8.  Unbalance  of  Communication  Circuits. 

Differences  in  the  admittances  to  ground  or  series  impedances  of  the 
Iwo  conductors  of  a  metallic  communication  circuit  cause  currents  in 
its  terminal  apparatus  when  a  voltage  is  induced  between  its  conductors 
and  ground  or  along  its  conductors  in  multiple.  These  unbalances 
may  be  reduced  to  the  smallest  practicable  values  by  transposing  the 
conductors  and  by  proper  design,  construction  and  maintenance  of 
open-wire  lines,  cables  and  connected  apparatus.  A  small  amount  of 
unbalance  is,  of  course,  unavoidable.  Since  the  induced  currents  here 
considered  are  proportional  to  the  product  of  the  unbalance  and  the 
induced  voltage,  it  is  necessary  to  restrict  the  amounts  of  either  or 
both  these  factors  in  order  to  sufficiently  limit  the  induced  currents  in 
the  terminal  apparatus  of  metallic  telephone  circuits. 

9.  Transients  and  Abnormal  Conditions, 

When  a  section  of  power  circuit  is  energized  or  de-energized  a  sudden 
change  takes  place  in  the  electric  and  magnetic  fields  about  the  cir- 
cuit. If  the  several  conductors  are  not  energized  or  de-energized  at 
exactly  the  same  instant,  large  residual  voltages  and  currents  exist 
momentarily.  An  extreme  case  of  this  sort  occurs  when  single-pole 
switches  are  operated  successively,  or  when  one  pole  of  a  switch  fails 
to  operate. 

When  one  conductor  becomes  grounded  there  is  a  sudden  change 
from  a  condition  of  approximate  balance  to  one  of  large  unbalance 
which  persists  until  the  circuit  is  de-energized  or  the  fault  cleared. 

At  the  time  of  such  abnormal  conditions  of  power  circuits  the  induc- 
tion in  parallel  communication  circuits  is  greatly  in  excess  of  that 
experienced  under  conditions  of  normal  operation,  sometimes  causing 
hazardous  voltages,  and  acoustic  shocks  to  telephone  users.  If  the 
protective  devices  of  communication  circuits  are  operated,  service  inter- 
ruption continues  for  a  considerable  period  after  the  initial  cause  has 
subsided,  until  such  devices  are  restored.  Where  telephone  circuits  are 
affected  the  operating  personnel  may  be  temporarily  demoralized  by 
severe  acoustic  shocks.  An  "arcing  ground"  on  a  power  circuit  not 
normally  connected  to  ground  may  continue  for  sonic  lime  with  constant 
repetition  of  the  accompanying  transients,  and  corresponding  severe 
disturbance. 


28  FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE 

The  means  sometimes  employed  in  handling  faults  in  power  circuits 
of  repeatedly  re-energizing  the  faulty  circuit,  either  to  burn  off  a 
"ground"  or  locate  it  by  sectionalization,  often  greatly  aggravates  the 
disturbance  to  communication  circuits  by  repetition. 

Abnormal  conditions  and  severe  switching  transients  are,  aside  from 
their  detrimental  effects  on  communication  circuits,  very  undesirable 
from  the  standpoint  of  power-system  operation.  The  frequency  of 
their  occurrence  can  only  be  lessened  by  high-grade  design  and  con- 
struction and  by  careful  operation  and  maintenance. 

10.  Nonessential  Features  Cause  Greatest  Interference. 

It  will  be  apparent  from  the  foregoing  that  those  features  of  power 
and  communication  circuits,  which  have  the  greatest  tendency  to  result 
in  interference,  while  not  wholly  avoidable,  are  nonessentials  which 
serve  no  useful  end  in  the  normal  functioning  of  the  circuits,  and  that 
the  necessary  precautions  for  the  prevention  of  inductive  interference 
are  not  incompatible  with  a  high  grade  of  service  but  to  some  extent 
further  that  end.  This  circumstance  is  fortunate  alike  for  the  public, 
the  power  companies  and  the  communication  companies. 

GUIDING  PRINCIPLES  FOR  PREVENTING   INTERFERENCE. 

The  following  are  the  basic  physical  principles  which  underlie  the 
rules  recommended  in  Part  Three  and  which  should  guide  all  efforts  to 
prevent  inductive  interference. 

1.  Avoidance  of  close  proximity. 

By  no  other  means  can  complete  freedom  from  interference  be 
secured. 

2.  Elimination  or  suppression  of  harmonics. 

To  the  existence  of  harmonics  is  due  practically  all  interference  to 
telephone  circuits  under  the  normal  operating  conditions  of  parallel 
power  circuits.  Improvement  in  this  respect  may  be  effected  by  giving 
due  regard  to  its  importance  in  the  purchase  of  new  equipment. 

3.  Limitation  of  residuals. 

The  intensity  of  the  induction  due  to  residual  voltages  and  currents 
is  relatively  more  severe  than  that  due  to  balanced  voltages  and  cur- 
rents and  induction  arising  from  residuals  cannot  be  neutralized  by 
power  transpositions.  They  can  be  lessened  by  balancing  the  line  and 
also  the  load,  by  the  use  of  advantageous  transformer  connections  and 
by  the  avoidance  of  excessive  magnetic  density  in  the  iron  cores  of 
transformers. 

4.  Reduction  of  intensity  of  induction  by  favorable  arrangements  of  conductors. 

Within  the  latitude  afforded  by  various  practical  configurations  of 
power  and  communication  circuits  the  induction  with  some  arrange- 
ments is  of  much  less  intensity  than  with  others.  In  cases  of  multi- 
circuit power  lines  important  advantage  can  be  secured  by  care  in  fixing 
the  phase  relations  of  the  conductors  of  the  several  circuits. 


FINAL    REPORT    ON    INDI'CTIVK   INTERFERENCE  29 

5.  Neutralization  of  induction  by  co-ordinated  transposition  systems. 

By  means  of  transpositions  in  both  classes  of  circuits  within  a  parallel 
the  phase  or  direction  of  the  induction  may  be  controlled  so  that 
mutually  neutralizing  effects  are  created  in  neighboring  lengths  of  cir- 
cuit. To  be  effective  the  transposition  systems  of  the  two  classes  of 
lines  must  be  co-ordinated. 

6.  Balancing  of  metallic  communication  circuits. 

Accurate  balancing  of  metallic  communication  circuits,  particularly 
telephone  circuits,  tends  to  reduce  the  disturbing  effect  of  induction 
from  parallel  power  circuits  and  other  near-by  communication  circuits. 
Unbalances  are  reduced  by  transposing  the  conductors  and  by  careful 
design,  construction  and  maintenance  of  lines  and  apparatus. 

7.  High-grade-construction  and  care  in  the  operation  and  maintenance  of  power 

circuits. 

No  means  are  known,  except  increased  separation  of  the  two  classes 
of  lines,  whereby  the  severe  momentary  disturbances  to  communication 
circuits,  due  to  abnormal  conditions  on  neighboring  power  circuits,  can 
be  prevented;  hence  the  importance  of  minimizing  such  occurrences  by 
high  standards  of  construction,  operation  and  maintenance. 


30  FINAL   REPORT   OK   INDUCTIVE   INTERFERENCE 

PART  THREE. 

REVISED   RULES   RECOMMENDED  BY   COMMITTEE. 
Reasons  for  Revising  Rules. 

Since  submitting  its  preliminary  report,  dated  July  7,  1914,  recom- 
mending provisional  rules  for  the  prevention  or  mitigation  of  inductive 
interference  which  were  later  embodied  in  General  Order  No.  39,  this 
Committee  has  greatly  extended  its  investigations  of  some  important 
branches  of  the  subject.  Considerable  experience  in  the  practical  appli- 
cation of  the  rules  has  also  been  gained.  In  the  light  of  the  additional 
information  thus  made  available  and  with  due  consideration  of  criti- 
cisms and  suggestions  which  have  been  offered  by  others,  the  Committee 
has  formulated,  and  herewith  presents,  revised  rules  which  it 
recommends  be  embodied  in  a  new  order  of  the  Railroad  Commission 
to  supersede  General  Order  No.  39. 

In  formulating  these  revised  rules,  the  Committee  has  endeavored  to 
utilize  the  information  obtained  since  its  former  report  so  that  the 
rules  may  be,  so  far  as  practicable,  definite  and  authoritative  in  respect 
to  the  specific  limitations.  The  general  arrangement  of  the  rules  hn^ 
been  modified  in  order  to  better  meet  the  requirements  of  practical  use. 
A  detailed  discussion  of  such  of  the  provisions  as  seem  to  require  it  is 
given  in  a  section  immediately  following  the  rules. 

Text  of  revised  rules. 

RULES  GOVERNING  THE  CONSTRUCTION  AND  OPERATION  OF 
POWER  AND  COMMUNICATION  LINES  FOR  THE  PREVEN- 
TION OR  MITIGATION  OF  INDUCTIVE  INTERFERENCE. 

I.     GENERAL  PROVISIONS. 

(a)  Applicability  of  rules. 

These  rules,  except  as  otherwise  provided  in  I  (e)  shall  apply  and  be 
effective  as  follows : 

1.  Rules  limited  to  lines  involved  in  a  parallel,*  or  to  apparatus  con- 
nected to  such   lines,   shall   apply   only   in   case   of   parallels   created 
hereafter;  except  that  rules  relating  to  operation  or  maintenance  shall 
apply  to  all  such  lines  and  apparatus,  both  existing  and  new. 

2.  Rules  not  limited  to  lines  involved  in  a  parallel,  or  to  apparatus 
connected  to  such  lines,  shall  apply  to  new  construction  only,  including, 
however,  existing  lines  and  apparatus  when  such  are  generally  recon- 
structed or  renewed. 

(b)  Co-operation. 

Any  party  contemplating  new  construction  which  may  create  a 
parallel  shall  confer  with  the  other  party  or  parties  concerned  and  they 
shall  co-operate  with  a  view  of  avoiding  the  parallel,  or,  if  this  be 
impracticable,  of  minimizing  the  resulting  interference.  Failure  to 
comply  with  this  requirement  will  receive  consideration  by  this  Com- 
mission in  any  subsequent  issue  involving  such  construction. 

*For  definition  of  "parallel"  see  page  32. 


FINAL    REPORT    ON    1NI>r<'TlYK    INTERFERENCE  3.1 

(c)  Principle  of  least  cost. 

When  there  are  two  or  more  different  practicable  methods  of  avoiding 
or  mitigating  interference,  the  method  which  involves  the  least  total 
cost  shall  in  general  be  adopted  irrespective  of  whether  the  necessary 
changes  are  made  in  the  plant  of  the  party  creating  the  parallel  or  in 
the  plant  of  the  other  party ;  provided,  however,  that  preference  shall 
In-  uiven  to  methods  of  avoiding  a  parallel  over  methods  of  mitigating 
interference;  and  provided,  further,  that  as  between  different  methods 
of  mitigation  having  different  degrees  of  effectiveness,  the  most  effective 
method,  the  cost  of  which  can  be  justified,  shall  be  adopted.  In  esti- 
mating such  costs,  all  factors  of  expense  to  both  parties  shall  be  taken 
into  account. 

(d)  Existing  parallels. 

Parties  operating  power  or  communication  lines  shall  exercise  due 
diligence  in  applying  measures,  in  general  accordance  with  the  prin- 
ciples of  these  rules,  for  mitigating  inductive  interference  due  to  exist- 
ing parallels.  Any  such  parallels  which  now  or  hereafter  cause 
excessive  interference  shall  be  attended  to  promptly. 

When  lines  involved  in  existing  parallels  are  added  to,  extended  or 
generally  reconstructed,  or  when  additional  apparatus  is  connected  to 
such  lines,  or  when  apparatus  now  connected  to  such  lines  is  renewed 
or  rearranged,  the  new  or  changed  plant  shall  thereafter  conform  to  the 
provisions  of  these  rules. 

(e)  Saving  clause. 

The  Commission  reserves  the  right  to  ijiodify  any  of  the  provisions 
of  these  rules  in  specific  cases,  when  in  the  Commission's  opinion,  public, 
interest  would  be  served  by  so  doing. 

II.     DEFINITIONS. 

Certain  technical  terms  are  employed  herein  in  the  senses  set  forth 
in  the  following  definitions: 

(a)  Class  H  power  circuit. 

The  term  "Class  H  Power  Circuit"  means  any  overhead  open-wire 
constant-potential  alternating-current  power  transmission  or  distribu- 
tion circuit  or  electrically  connected  network  which  has  5,000  volts  or 
more  between  any  two  conductors  or  2,900  volts  or  more  between  any 
conductor  and  ground;  except  railway  trolley  circuits  and  feeders 
electrically  connected  therewith. 

(b)  Electrically   connected. 

The  term  "Electrically  Connected"  means  connected  by  a  conduct- 
ing path  or  through  a  condenser,  as  distinguished  from  connection 
merely  through  magnetic  induction. 

(c)  Signal   circuit. 

The  term  "Signal  Circuit"  means  any  telephone,  telegraph,  mes- 
senger call,  clock,  fire,  police  alarm,  or  other  circuit  of  similar  nature 
used  exclusively  for  the  transmission  of  signals  or  intelligence,  which 


32  FINAL   REPORT   ON   INDUCTIVE  INTERFERENCE 

operates  at  less  than  400  volts  to  ground,  or  750  volts  between  any  two 
points  of  the  circuit,  provided  that  if  the  voltage  exceeds  150,  the  power 
transmitted  shall  not  exceed  150  watts. 

(d)  Communication   circuit. 

The  term  "Communication  Circuit"  means  any  overhead  open-wire 
signal  circuit,  except  that,  if  such  circuit  be  a  telephone  circuit,  it  is 
limited  to  inter-exchange  metallic  telephone  circuits  and  to  metallic 
telephone  circuits  operated  by  a  railroad  or  other  company  for  dispatch- 
ing purposes,  or  for  public  use  between  separate  communities. 

(e)  Line. 

The  term  "Line"  means  any  circuit  or  aggregation  of  circuits  car- 
ried on  poles  or  towers,  and  includes  the  supporting  elements. 

(f)  Parallel. 

The  term  "Parallel"  means  a  condition  where  a  Class  H  Power  cir- 
cuit and  a  communication  circuit  follow  substantially  the  same  course 
or  are  otherwise  in  proximity  for  a  sufficient  distance  so  that  the  powrer 
circuit  is  liable  to  create  inductive  interference  in  the  communication 
circuit. 

With  some  parallels  interference  occurs  only  at  times  of  abnormal 
conditions  on  the  power  circuit  in  which  case  such  of  these  rules  as 
affect  induction  only  under  normal  operating  conditions  do  not  apply. 
When  the  application  of  any  rule  is  thus  restricted,  the  condition  under 
which  the  rule  applies  is  referred  to  as  a  "normal"  parallel. 

(g)  Configuration. 

The  term  "Configuration"  means  the  geometrical  arrangement  of  a 
circuit  or  circuits,  including  the  size  of  the  wires,  and  their  relative? 
positions  with  respect  to  one  another  and  earth. 

(h)   Transposition. 

The  term  "Transposition"  denotes  an  interchange  of  position  of  the 
conductors  of  a  circuit  between  successive  lengths  thereof. 

(i)    Barrel. 

The  term  "Barrel"  means  an  arrangement  of  a  section  of  power  cir- 
cuit of  uniform  configuration  within  which  each  conductor  occupies 
each  of  the  conductor  positions  for  equal  distances. 

(j)    Discontinuity. 

The  term  "Discontinuity"  means  any  abrupt  change  in  the  relative 
positions  of  a  power  and  a  communication  circuit,  or  any  abrupt  change 
in  configuration,  line  impedance  or  load  along  either  such  circuit 
(including  such  changes  due  to  connected  circuits,  transformers,  cables, 
loading  coils  or  other  apparatus)  which  materially  affects  the  magni- 
tude or  phase  of  the  induced  voltages  or  currents  per  unit  length  or 
the  capacitances  of  either  circuit.  Transpositions,  however,  are  not 
considered  to  be  discontinuities. 


FINAL    RKl'ORT    ON    INDUCTIVE   INTERFERENCE  33 

(k)   Co-ordination. 

The  term  "Co-ordination"  as  applied  to  transposition  systems  menus 
that  the  transpositions  in  power  and  communication  circuits  involved 
in  a  parallel  are  efficiently  located,  with  respect  to  each  other  and  to 
the  discontinuities,  for  reducing  the  inductive  effects  on  the  communi- 
cation circuits. 

(I)    Balanced   and    residual   voltages. 

The  voltages  to  tiround  of  the  several  wires  of  a  power  circuit  arc 
divided  for  convenience  into  two  classes  of  components,  "balanced" 
and  "residual." 

The  "balanced  voltages"  are  those  components  which  are  equal  in 
magnitude  and  have  such  phase  relations  that  their  algebraic  sum  is 
zero  at  ever}-  instant. 

The  remaining  components  of  the  voltages  to  ground,  which  exist 
under  conditions  other  than  perfect  balance,  are  termed  residual.  They 
are  equivalent  to  a  single-phase  voltage  impressed  between  the  power 
wires  in  multiple  and  ground.  The  sum  of  the  residual  components  is 
termed  the  "residual  voltage1"  of  the  circuit.  In  case  of  a  three-phase 
circuit  it  is  three  times  the  equivalent  single-phase  voltage  above 
mentioned. 

Mathematically  expressed,  the  residual  voltage  is  the  vector  sum  of 
the  voltages  to  ground  of  the  several  wires  of  a  power  circuit,  while 
the  balanced  voltages  aro  those  components  whose  vector  sum  is  zero. 

(m)    Balanced  and  residual  currents. 

The  currents  in  the  several  wires  of  a  power  circuit  are  divided  for 
convenience  into  two  class  of  components,  "balanced"  and  "residual." 

The  "balanced  currents"  are  those  wholly  confined  to  the  wires  of  the 
circuit.  Hence  their  algebraic  sum  is  zero  at  every  instant. 

The  remaining  components  of  the  currents  in  the  several  wires,  which 
exist  under  conditions  other  than  perfect  balance,  are  termed  residual. 
The  sum  of  the  residual  components  is  the  "residual  current"  of  the 
circuit.  It  is  equivalent  to  a  single-phase  current  in  a  circuit  having 
the  power  wires  in  multiple  as  one  side,  and  ground  as  the  other  side. 

Mathematically  expressed,  the  residual  current  is  the  vector  sum  of 
the  currents  in  the  several  power  wires  while  the  balanced  currents  are 
those  components  whose  vector  sum  is  zero. 

III.     LOCATION   OF   LINES. 

(a)  Avoidance  of  parallels. 

Every  reasonable  effort  shall  be  made  to  avoid  creating  parallels.  If 
the  parties  concerned  can  agree  upon  a  plan  for  providing  an  adequate 
separation  of  the  two  classes  of  lines  so  as  to  avoid  interference,  such 
plan  shall  be  put  into  effect.  In  no  case  shall  a  parallel  be  created 
unless  the  cost  of  avoidance  by  separation  is  greater  than  the  cost  of 
the  remedial  measures  required  by  these  rules. 

(b)  Notice  of  intention. 

The  party  proposing  to  build  a  new  Class  H  power  or  a  communica- 
tion line  which  will  create  a  parallel,  or  generally  to  reconstruct  or 


34  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

change  the  operating  conditions  of  an  existing  line  involved  in  a  par- 
allel, shall  give  due  notice  (at  least  sixty  days  where  practicable  but  in 
any  event  not  less  than  twenty  days  in  advance  of  construction,  except 
for  minor  extensions,  for  which  notice  shall  be  given  immediately  after 
the  work  is  authorized)  of  such  intention  to  the  other  party  including 
full  information  as  to  the  location  within  the  parallel  and  such  other 
features  of  the  proposed  line  as  would  affect  induction. 

(c)  Distance  between   lines. 

Class  II  power  lines  and  communication  lines  shall  be  kept  as  far 
apart  as  practicable.  Their  separation  should  be  at  least  equal  to  the 
height  above  ground  of  the  power  wires,  except  when  closer  proximity 
is  unavoidable. 

If,  in  any  case  of  inductive  interference,  it  should  be  found  imprac- 
ticable to  obtain  a  proper  degree  of  relief  by  means  of  the  remedial 
measures  set  forth  in  these  rules  or  by  other  measures  of  a  remedial 
nature,  the  parties  concerned  shall  agree  upon  and  put  into  effect  a 
plan  for  increasing  the  separation  of  the  lines  within  the  parallel. 

To  promote  the  effective  application  of  transpositions,  both  parties 
shall  endeavor  to  maintain  a  uniform  .separation  of  the  two  lines 
throughout  each  normal  parallel.  However,  in  general,  when  it  is 
feasible  to  secure  more  than  a  20  per  cent  increase. in  separation,  for  a 
distance  in  excess  of  one  mile,  this  shall  be  done. 

(d)  Length   of  parallels. 

Parallels  shall  be  made  as  short  as  practicable. 

(e)  Discontinuities. 

In  the  location,  construction  and  general  reconstruction  of  lines 
within  normal  parallels  every  reasonable  effort  shall  be  made  to  avoid 
discontinuities  (except  those  due  to  increases  in  separation  as  provided 
for  in  (c)  above)  which  would  interfere  with  the  application  of  effective 
and  economical  co-ordinated  transposition  systems  in  the  power  and 
communication  lines. 

In  the  location  and  construction  of  the  first  line  along  a  public  high- 
way, special  effort  shall  be  made  to  avoid  crossing  the  highway  and 
also  to  avoid  other  features  which  would  result  in  unnecessary  discon- 
tinuities in  the  event  of  the  construction  of  another  line  along  the  same 
highway. 

IV.     DESIGN   AND  CONSTRUCTION   OF   LINES. 

(a)  General   requirements. 

The  quality  of  material,  workmanship,  methods  and  grade  of  con- 
struction shall  be  in  accordance  with  approved  modern  practice  with 
special  regard  to  the  prevention  of  failures  and  the  avoidance  of 
features,  such,  for  example,  as  inferior  insulation,  which  would  tend  to 
cause  or  promote  inductive  interference. 

(b)  Arrangement  and  spacing  of  power  conductors. 

In  the  design  for  construction  or  general  reconstruction  of  Class  H 
power  lines,  consideration  shall  be  given  to  the  configuration  of  the  lines 


FINAL    REPORT   ON   INDUCTIVE   INTERFERENCE  35 

with  a  view  to  minimizing  (1)  throughout  the  entire  length  of  the  line 
inequalities  among  the  capacitances  to  earth  of  the  conductors;  and  (2) 
within  normal  parallels  the  intensity  of  the  inductive  effects.  When 
two  or  more  circuits  are  carried  on  one  line  the  phase  relations  among 
the  conductors  of  the  different  circuits  should  he  chosen  with  the  same 
purposes  in  view.  The  configurations  to  be  preferred  for  three-phase 
lines  under  different  conditions  are  discussed  in  the  Exhibit  attached 
hereto. 

Excessive  spacing  of  conductors  should  be  avoided. 

Two-wire  branches  electrically  connected  to  a  three-phase  Class  1 1 
power  circuit  should  be  avoided  except  those  so  short  that  they  do  not 
materially  unbalance  the  three-phase  circuit.  Where  such  branches  are 
employed  they  should  be  so  distributed  as  to  cause  minimum  unbalance. 

No  single-wire  grounded  Class  H  power  circuits  or  branches  of  multi- 
wire  Class  H  power  circuits  shall  be  employed. 

(c)   Transpositions — General. 

All  Class  II  power  circuits  and  metallic  communication  circuits,  or 
extensions  of  such  circuits,  hereafter  constructed  or  generally  recon- 
structed, shall  be  transposed  throughout  their  entire  lengths  in  such 
manner  as  to  balance,  as  nearly  as  practicable,  the  capacitances  to  earth 
of  their  conductors.  For  single-circuit  three-phase  lines  the  maximum 
length  of  barrel  for  this  purpose  shall  be  twelve  miles  for  circuits  of 
triangular*  configuration  and  six  miles  for  other  configurations.  For 
twin-circuit  three-phase  lines  the  maximum  length  of  barrel  shall  be  six 
miles ;  except  that  for  circuits  of  the  vertical  type  (including  cases 
with  the  middle  conductors  displaced  slightly  outward)  and  the 
equilateral  triangular  type  with  vertices  upward,  nine-mile  barrels 
may  be  used  when  the  circuits  are  interconnected  for  minimum  unbal- 
ances. The  accompanying  Exhibit  contains  information  concerning  the 
methods  of  interconnection  giving  minimum  unbalances. 

Exceptions.  Power  lines,  located  principally  on  private  rights  of  way  and  not 
electrically  connected  to  other  lines,  are  exempt  from  this  rule  if  separated  from 
existing  communication  lines,  and  from  highways  required  for  the  future  construc- 
tion of  communication  lines,  by  distances  not  less  than  those  given  below,  except  for 
crossings  at  angles  over  30  degrees  and  other  sections  of  unavoidable  closer  proximity 
not  exceeding  one  mile  in  total  length  in  each  ten  consecutive  miles  of  line,  pro- 
vided, however,  that  such  sections  of  closer  proximity  to  any  one  such  communica- 
tion line  or  highway  shall  not  exceed  one  mile  in  each  thirty  consecutive  miles  of  line. 


Voltage  between  power  conductors 


Minimum 
separation 
from  high- 
ways and 
communica- 
tion lines 


Below  50,000 600  feet 

50,000-  75,000  _J i  750  feet 

75,000-100,000  !  850  feet 

100,000-1.50,000    1,000  feet 

150,000-200,000    _  1,200  feet 


For  power  lines  meeting  all  these  conditions  for  exemption  except  that  they  are 
electrically  connected  to  other  lines  through  autotransformers.  the  maximum  lengths 
of  barrel  may  be  twice  those  specified  above. 


*A  triangular  configuration  as  here  used  means  one  in  which  the  altitude  of  the 
triangle  exceeds  one-half  the  length  of  the  longest  side  as  base. 


36  FINAL   REPORT    ON   INDUCTIVE   INTERFERENCE 

The  question  of  whether  highways  that  may  be  involved  will  be  required  for  future 
communication  lines  shall  be  settled  by  agreement  between  the  power  company 
contemplating  construction  and  the  communication  companies  operating  within  the 
territory  to  be  traversed.  In  the  event  of  disagreement  or  if  there  is  no  such 
communication  company,  the  matter  shall  be  referred  to  this  Commission.  In  cases 
where  the  proposed  use  of  a  particular  highway  by  a  communication  company  would 
be  the  determining  factor  in  deciding  whether  a  given  power  line  must  be  transposed, 
such  communication  company  shall  make  an  effort  to  locate  its  proposed  line  else- 
where and  the  decision  shall  be  made  in  accordance  with  the  principle  of  least  cost 
laid  down  in  I  (c). 

Existing  Class  H  power  circuits  and  those  exempted  under  the  pre- 
ceding paragraph,  which  hereafter  become  involved  in  normal  parallels, 
shall  be  transposed  so  as  to  balance  their  capacitances  to  earth,  when 
necessary  for  limiting  residual  voltages  and  currents  to  amounts  which 
can  be  tolerated.  The  location  and  number  of  transpositions  for  this 
purpose  shall  be  determined  by  agreement  of  the  parties  concerned. 

In  the  location  and  spacing  of  the  transpositions  due  regard  shall  be 
paid  to  discontinuities  which  affect  the  capacitances  of  the  circuit. 
Sections  of  circuit  between  such  points  of  discontinuity  should  be 
treated  independently. 

In  general,  transpositions  should  be  omitted  at  the  junction  points  of 
successive  barrels. 

Metallic  communication  circuits,  and  single-phase  and  two-phase 
"Class  II  power  circuits,  shall  be  transposed  at  intervals  not  exceeding 
four  miles. 

Power  circuits  less  than  three  miles  in  length  are  not  required  to  be 
transposed  outside  of  parallels,  except  when  the  absence  of  transposi- 
tions would  materially  impair  the  balance  of  other  circuits  to  which 
they  are  electrically  connected. 

Power  circuits  with  grounded  neutrals,  having  a  voltage  of  less  than 
12,500  volts,  between  conductors,  are  not  required  to  be  transposed  out- 
side of  parallels,  except  where  the  lack  of  such  transpositions  in  any 
specific  case  is  the  cause  of  interference. 

Within  normal  parallels  the  transpositions  in  the  two  classes  of  cir- 
cuits shall  be  as  provided  in  (d)  below.  When  the  transpositions 
required  in  a  parallel  impair  the  general  transposition  system  of  either 
line  outside  the  limits  of  the  parallel,  the  necessary  readjustment  of 
transpositions  shall  be  made  in  the  sections  of  line  adjacent  to  the 
parallel,  as  a  part  of  the  remedial  measures  therefor. 

(d)   Transpositions — Inside  limits  of  parallels. 

Within  each  normal  parallel  an  adequate  scheme  of  transpositions,  to 
neutralize,  so  far  as  practicable,  the  inductive  effects,  shall  be  installed 
in  the  power  circuits,  and  also  in  the  communication  circuits,  provided 
the  latter  are  metallic.  The  transposition  systems  in  the  two  classes  of 
circuits  shall  be  properly  co-ordinated.  The  parties  concerned  shall 
co-operate  to  determine  upon  the  transposition  scheme  to  be  employed. 
The  transpositions  required  in  the  line  last  constructed  shall  be  installed 
before  it  is  placed  in  service. 

In  applying  the  foregoing,  the  following  rules  shall,  in  general,  be 
observed : 


FINAL    REPORT   ON    INDUCTIVE    INTERFERENCE  37 

1.  For  cadi  normal   parallel  at  least   one  barrel  shall  be  installed  in 
the  power  circuit.     This  applies  also  to  a  section  of  parallel  where  it  is 
not  practicable  to  obtain   a   balance  by  combining  it  with  another  sec- 
tion.    In  applying  this  rule  it  is  not  intended  ordinarily  to  change  the 
span  lengths  required  for  other  purposes. 

2.  In  long  uniform  parallels  or  sections  of  parallel,  involving  a  tele- 
phone line  at  highway  separation  from  the  power  line,  the  barrels  shall 
be  three  miles  in  length,  subject  to  such  variation  as  may  be  necessary 
for   co-ordination    with    the    transpositions   required   in   the   telephone 
circuits.     Transpositions  should  in  general  be  omitted  at  the  junction 
points  of  successive  barrels. 

3.  Except  as  modified  by    (1)    above,   the  number  of  transpositions 
required  in  power  circuits  paralleling  telephone  circuits  shall  be  sub- 
ject   to  the  following  limitations  expressed   in   terms  of   the   average 
distance  between  successive  transpositions : 

For  power  circuits  of  50,000  volts  or  more  between  conductors, 
not  less  than  one  mile. 

For  power  circuits  of  less  than  50,000  volts  between  conductors, 
not  less  than  one-sixth  mile.* 

4.  In  case  of  a  parallel  between  a  power  line  and  a  telegraph  line  or 
other  grounded  communication  line,  the  transpositions  in  the  power  cir- 
cuit shall  be  located  with  due  regard  to  the  limits  of  the  parallels  and 
to  discontinuities,  in  order  to  form  as  nearly  as  practicable  a  balanced 
system,  subject  to  the  condition  that  the  transpositions  in  the  power 
circuit    are    not   required   to   be   less   than   one   mile   apart,   except   as 
modified  by   (1)   above.     In  lonu-  uniform  sections  of  parallel,  barrels 
six   miles   in    length   should    be   .sufficient.     Transpositions   should    be 
omitted  at  the  junction  points  of  successive  barrels. 

5.  The  question  of  the  most  economical  scheme  to  accomplish  the  pur- 
pose shall  always  be  considered.     Effort  shall  be  made  to  utilize  as  many 
as  practicable  of  the  existing  transpositions. 

It  is  suggested  that  in  case  of  a  short  section  of  new  line,  not  sufficient 
of  itself  to  require  transpositions,  but  which  is  likely  to  be  extended 
later  so  that  transpositions  would  then  be  necessary,  consideration  be 
given  to  the  advisability  of  installing  one  or  more  suitably  located  trans- 
positions in  the  new  section  of  line  while  it  is  being  constructed  in  order 
to  avoid  interrupting  the  service  by  adding  transpositions  afterwards. 

K.i'<'ci>tionx.  Cases  of  parallelism  may  occur  where  the  interference  is  due  almost 
wholly  to  residual  voltages  and  currents  in  which  event  transpositions  in  the  power 
circuit  are  not  required,  except  as  provided  in  IV  (c). 


V.      DESIGN,  CONSTRUCTION  AND  ARRANGEMENT  OF  APPARATUS. 
(a)    Quality  and  suitability. 

In  designing,  specifying,  or  otherwise  determining  the  quality  or  suit- 
ability of  apparatus  to  be  connected  to  Class  II  power  or  communication 

*While  barrels  of  approximately  three  miles,  as  provided  in  2  above,  are  generally 
to  be  employed,  the  shorter  barrels  specified  in  3  are  sometimes  necessary  in  short 
parallels  and  in  short  sections  of  parallels,  in  order  to  co-ordinate  with  the  discon- 
tinuities and  obtain  a  proper  degree  of  balance. 


38  FINAL    REPORT    ON    INDUCTIVE    INTERFERENCE 

circuits,  and  in  arranging  such  apparatus  for  use,  effort  shall  be  made  to 
avoid,  so  far  as  is  reasonably  practicable,  all  features  which  would  tend 
to  create  or  promote  inductive  interference  under  either  normal  or 
abnormal  conditions.  As  instances  in  applying  the  foregoing,  the  fol- 
lowing rules  shall  be  observed. 

(b)  Rotating  machinery. 

In  order  to  improve  conditions  generally,  companies  operating  Class 
II  power  circuits  shall  make  every  effort  to  minimize  the  high  frequency 
components  of  voltages  and  currents  caused  by  rotating  machinery.  All 
new  rotating  machinery  shall  have  as  nearly  as  practicable  a  pure  sine 
wave  of  voltage  and  shall  not,  in  any  case,  deviate  therefrom  to  exceed 
the  limit  set  forth  in  the  present  standardization  rules  of  the  American 
Institute  of  Electrical  Engineers. 

No  ground  connection  shall  be  used  on  the  armature  winding  of  an 
alternating-current  generator  or  motor  electrically  connected  to  a  power 
circuit  involved  in  a  normal  parallel  unless  means  are  employed  to 
avoid  unbalancing  the  circuit  and  to  reduce  triple-harmonic  residuals  as 
far  as  may  be  necessary  and  practicable. 

(c)  Transformers   and   their  connections. 

In  order  that  the  wave-shape  of  voltage  and  current  may  be  distorted 
as  little  as  practicable  by  transformers,  all  new  transformers  on  Class 
II  power  circuits  should  have  an  exciting  current  as  low  as  is  consistent 
with  good  practice,  and  which  shall  not,  at  rated  voltage,  exceed  10 
per  cent  of  the  full  load  current ;  except  that  for  transformers  without 
neutral  ground  connections  on  the  line  side,  the  exciting  current  at 
rated  voltage  need  not  be  less  than  0.2  ampere. 

Where  three-pha.se  transformers  are  employed  with  grounded  neutrals 
the  core  type  is  preferable  to  the  shell  type. 

Transformers  or  transformer  banks  shall  not  be  grounded  at  such 
points  of  their  windings  as  to  unbalance  a  connected  circuit  involved  in 
a  normal  parallel.  As  important  cases  under  this  rule,  no  grounded 
single-phase,  grounded  three-wire  two-phase,  or  grounded  open-star 
three-phase  connection  shall  be  so  employed. 

No  star-connected  transformers  or  autotransformers  shall  be  em- 
ployed with  a  grounded  neutral  on  the  side  connected  to  a  three-phase 
power  circuit  involved  in  a  normal  parallel,  unless  low-impedance 
delta-connected  secondary  or  tertiary  windings  or  other  equivalent 
means  are  used  for  suppressing  the  triple  harmonic  components  of  the 
residual  voltages  and  currents  introduced  by  the  transformers. 

Care  shall  be  taken  that  the  individual  units  in  each  grounded- 
neutral  bank  of  transformers,  connected  to  a  circuit  involved  in  a 
normal  parallel,  are  alike  as  to  type  and  rating,  including  all  electrical 
characteristics,  and  that  they  are  similarly  connected,  so  as  not  to 
unbalance  the  circuit. 

Closed-delta  connections  shall  be  used  wherever  practicable  in  prefer- 
ence to  open-delta  connections  on  three-phase  power  circuits  involved  in 
normal  parallels.  When  open-delta  connections  are  employed,  an  effort 
shall  be  made  to  distribute  such  connections  equally  among  the  three 
phases. 


J'MNAIj    K'KI'ORT    ON"    IN'WCTI  VK    INTMKKKKHNCE  «')!) 

Where  triple  harmonic  residual  voltages  and  currents  due  to  star- 
connected  transformer  hanks  exist  in  amounts  which  can  not  be 
tolerated,  and  it  is  inexpedient  to  isolate  the  transformer  neutrals,  such 
residuals  shall  be  limited  by  operating  the  transformers  at  reduced 
magnetic  density  or  by  other  available  means. 

(d)  Rectifiers. 

Rectifiers  and  other  apparatus  lending  to  distort  the  alternating  cur- 
rent wave  when  installed  on  power  lines  involved  in  normal  parallels, 
shall,  if  necessary,  be  equipped  with  suitable  auxiliary  apparatus  to 
prevent  harmful  distortion  of  the  wave-form  of  power-circuit  voltage 
or  current. 

(e)  Switches. 

Each  oil-break  switch  in  a  power-circuit  involved  in  a  parallel,  located 
between  the  source  or  sources  of  energy  and  the  parallel,  and  used  for 
energizing  or  de-energizing  the  circuit,  shall  have  all  poles  mechanically 
interconnected  for  simultaneous  action.  There  shall  be  at  least  one 
such  switch  so  located  as  to  control  the  supply  of  energy  to  each 
power  circuit  involved  in  a  parallel,  and,  except  at  stations  where 
an  operator  is  constantly  on  duty,  such  switch  shall  be  made  automatic 
for  short  circuits,  grounds,  and  in  case  of  grounded  neutral  circuits, 
for  abnormal  neutral  grounds. 

Careful  consideration  shall  be  given  to  means  of  minimizing  transient 
disturbances  caused  by  switching  operations  on  Class  H  power  circuits, 
which  would  cause  inductive  interference.  Wherever  practicable  pro- 
vision shall  be  made  for  switching  on  the  station-side  rather  than  on  the 
line-side  of  transformer  banks. 

Oil-break  switches,  having  their  poles  mechanically  interconnected  for 
simultaneous  action,  shall  be  provided  wherever  the  use  of  air  switches 
or  noninterconnected  single-pole  oil  switches  would  cause  harmful 
transient  disturbances  in  parallel  communication  circuits. 

(f)  Fuses. 

Switches  shall  be  used  instead  of  main  line  fuses  wherever  practicable! 
in  a  power  circuit  involved  in  a  parallel. 

(g)  Electrolytic   lightning  arresters. 

AVhen  electrolytic  lightning  arresters  are  employed  on  a  power  circuit 
involved  in  a  parallel  they  shall  be  equipped  with  auxiliary  charging 
resistances  and  contacts  so  arranged  that  the  horn  gaps  are  short- 
circuited  at  the  time  of  charging,  to  avoid,  as  far  as  possible,  the 
production  of  arcs. 

(h)    Special    instruments. 

Reliable  indicating  devices  shall  be  installed  at  the  source  of  supply 
of  power  circuits  involved  in  parallels,  to  inform  the  operators  imme- 
diately of  abnormal  conditions,  such  as  grounds,  and  wherever  possible, 
epen  circuits,  which  have  not  operated  automatic  switches. 

Whenever  a  neutral  Around  connection  is  employed  on  a  circuit 
involved  in  a  parallel,  an  ammeter,  suitable  for  measuring  the  current 
in  the  neutral  under  normal  operating  conditions,  shall  be  installed  in 


40  FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE 

each  neutral  connection  to  ground  at  the  main  generating  and  main 
attended  substations  on  the  power  system  electrically  connected  to  the 
circuit  involved  in  the  parallel. 

(i)    Communication  apparatus. 

All  apparatus  electrically  connected  to  metallic  communication  cir- 
cuits involved  in  parallels  shall  be  designed  and  constructed  so  as  to 
secure  as  nearly  as  practicable  an  accurate  balance  of  the  series 
impedances  and  the  admittances  to  earth  of  the  two  sides  of  the  cir- 
cuits in  order  to  minimize  the  detrimental  effects  of  induction  from 
parallel  power  circuits. 

VI.     OPERATION   AND   MAINTENANCE. 

(a)  General  requirements. 

Power  and  communication  companies  shall  use  all  reasonable  means 
to  operate  and  maintain  circuits  involved  in  parallels  in  such  a  manner 
as  to  minimize  interference  under  conditions  of  normal  operation,  and 
to  avoid  transient  disturbances. 

(b)  Balance. 

Jn  the  maintenance  of  both  power  and  communication  circuits 
involved  in  parallels  special  care  shall  be  given  to  the  prevention  of 
mechanical  and  electrical  failures  which  would  cause  or  promote 
transient  disturbances  or  unbalances  such  as  those  due  to  tree-grounds, 
defective  or  dirty  insulators  or  other  faults. 

The  voltages  and  currents  of  power  circuits  involved  in  parallels  shall 
be  kept  balanced  as  closely  as  practicable  and  accidental  unbalances 
shall  be  promptly  corrected. 

(c)  Record  of  neutral   current. 

At  all  points  on  grounded  neutral  systems  equipped  as  required  in  V 
(h),  the  power  company  shall  observe  and  record  daily  the  approximate 
maximum  neutral  current. 

(d)  Transformers. 

No  transformers  connected  to  power  circuits  involved  in  normal 
parallels  shall  be  operated  at  more  than  10  per  cent  above  their  rated 
voltage.  Wherever  practicable  in  case  of  existing  equipment  and  in  all 
cases  of  new  equipment,  transformer  banks  with  grounded  neutrals  on 
the  side  which  is  connected  to  a  power  circuit  involved  in  a  normal 
parallel  shall  not  be  operated  at  more  than  5  per  cent  above  their  rated 
voltage. 

(e)  Switching. 

In  all  switching  operations  care  shall  be  taken  to  avoid,  so  far  as 
possible,  the  production  of  harmful  transient  disturbances. 

(f)  Charging  electrolytic  lightning  arresters. 

When,  notwithstanding  compliance  with  V  (</),  interference  is  caused 
by  charging  electrolytic  lightning  arresters,  such  charging  shall  be  done 
at  night,  so  far  as  is  possible,  preferably  between  2  a.m.  and  4  a.m. 


FINAL    RKI'OKT    o.V    INHrCTIVK    I  NTIW  FKK'KNf  '1 1  41 

(g)    Abnormal  conditions. 

Power  companies  shall  adopt  operating  rules  which  shall  specifically 
outline  the  procedure  for  their  operators  during  times  when  a  power 
circuit,  involved  in  a  parallel  is  abnormally  unbalanced,  as  will  occur 
with  an  open,  grounded  or  short-circuited  line  or  transformer  winding. 

Such  rules  shall  in  general  provide  for  the  discontinuance  of  opera- 
tion of  the  power  line  until  the  fault  is  remedied,  excepting  only  those 
cases  where  it  is  clear  that  the  service  rendered  the  public  by  con- 
tinuing operation  of  this  section  of  power  line  is  of  greater  importance 
than  the  communication  service  interrupted  by  such  continued 
operation. 

When  it  is  necessary  to  energize  a  defective  power  line  in  order  to 
locate  a  fault,  care  shall  be  taken  to  avoid,  as  far  as  possible,  repeatedly 
energizing  any  section  of  such  line  which  parallels  communication 
circuits,  until  the  fault  has  been  cleared.  Whenever  possible  the  faulty 
section  of  line  shall  not  be  energized  more  than  once  until  disconnected 
from  the  section  of  line  involved  in  the  parallel. 

To  facilitate  the  study  and  prevention  of  disturbances  in  communi- 
cation circuits,  occasioned  by  transient  conditions  of  power  circuits, 
accurate  record  shall  be  kept  of  the  nature  and  time  of  occurrence  of 
failures,  changes  in  operating  arrangements  and  all  switching  during 
times  of  abnormal  conditions  of  Class  II  power  circuits  involved  in 
parallels;  and  of  all  transient  disturbances  in  communication  circuits. 
These  records  shall  be  made  available  for  use  in  tracing  the  causes  «-!' 
such  transient  disturbances. 


EXHIBIT. 

ARRANGEMENT  AND  SPACING  OF  POWER  CONDUCTORS. 
Supplementing  IV  (b)  and  IV  (c). 

The  arrangement  and  spacing  of  the  conductors  of  power  circuits  are 
of  importance  in  determining  (1)  the  unbalances  or  inequalities 
among  the  capacitances  of  the  conductors  to  ground,  which  cause 
residual  voltages  and  currents,  and  (2)  the  intensity  of  the  inductive 
effects  produced  in  communication  circuits  by  the  balanced  voltages 
and  currents  of  parallel  power  circuits.  For  sections  of  line  within 
limits  of  parallels,  consideration  of  the  inductive  effects  should  in  gen- 
eral control  rather  than  consideration  of  the  capacitance  unbalances. 
For  sections  of  line  outside  the  limits  of  parallels,  consideration  of 
capacitance  unbalances  should  be  given  the  greater  weight,  particularly 
for  circuits  operated  without  grounded  neutrals. 

The  figures  and  comparisons  given  herein  apply  to  nontransposed  cir- 
cuits, but  the  comparisons  of  different  configurations  hold  also  for 
transposed  circuits,  provided  the  circuits  are  transposed  identically. 
If  there  were  no  irregularities  or  inexactnesses  to  impair  the  effective- 
ness of  a  transposition  system,  it  would  be  possible  theoretically, 
neglecting  the  effect  of  phase  change  and  attenuation,  to  obtain  a 
perfect  balance  by  means  of  transpositions,  irrespective  of  the  arrange- 
ment of  the  conductors.  Practically,  however,  circuits  even  when 


42  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

carefully  transposed  have  a  material  resultant  unbalance,  particularly 
at  the  frequencies  of  the  higher  harmonics,  and  this  unbalance  is 
proportional  to  the  unbalance  characteristic  of  the  circuit  configuration. 
In  a  similar  manner  the  resultant  induction  due  to  a  power  circuit  is 
proportional  to  the  intensity  of  the  induction  characteristic  of  the 
configuration.  Configurations  differ  widely  in  respect  to  their  char- 
acteristic unbalances  and  intensities  of  induction,  some  arrangements, 
particularly  of  twin  circuits,  giving  fully  90  per  cent  less  unbalances 
or  induction  than  others. 

The  effects  of  the  arrangement  and  spacing  of  conductors  on  the 
unbalances  of  their  capacitances  to  ground  and  on  the  induction  pro- 
duced in  parallel  communication  circuits  are  discussed  separately. 

EFFECT  ON  CAPACITANCE  UNBALANCE. 

In  general,  the  capacitances  to  ground  of  the  conductors  of  a  non- 
transposed  multiconductor  circuit  are  unequal,  the  magnitude  of  the 
percentage  unbalances  being  determined  by,  and  therefore  characteristic 
of,  the  configuration  of  the  circuit.  This  "characteristic  unbalance"  is 
an  important  factor  in  determining  the  residual  voltage  of  a  circuit 
isolated  from  ground,  and  in  determining  the  residual  current  of  a 
grounded  neutral  circuit,  in  so  far  as  such  current  is  caused  by  the  line 
itself.  Taking  as  a  measure  of  the  characteristic  unbalance,  the 
residual  voltage  of  a  short,  uniform,  nontransposed  circuit  without 
metallic  connection  to  ground  and  energized  with  balanced  three-phase, 
voltages  between  conductors,  termed  the  "characteristic  residual  volt- 
age," the  following  table  affords  a  comparison  of  various  configurations 
of  single-circuit  powTer  lines  over  the  practical  range  of  cross-sectional 
dimensions. 

Characteristic  Residual  Voltage;  Per  Cent  of  Balanced  Three-phase  Voltage 
Between   Conductors. 

Configuration 

Equilateral  triangle 0.5 to    4 

Vertical   6    to  11 

Horizontal- 
Symmetrical   5    to    9 

Unsymmetrical   7    to  11 

Isosceles  triangle- 
Base  horizontal 0    to    8 

Base  vertical  0.5  to    9 

"L"    2    to    6 

Inverted  "L" 4    to    7 

Triangular  circuits  have  the  smallest  unbalances  and  characteristic 
residual  voltages.  Symmetrical  horizontal  and  vertical  circuits  are 
about  alike,  the  vertical  having  slightly  the  greater,  and  unsymmetrical 
horizontal  circuits  have  the  largest.  The  characteristic  residual  volt- 
ages of  symmetrical  horizontal  and  vertical  configurations  are  from  2 
to  8  times  that  of  a  corresponding  equilateral  triangular  circuit, 
depending  upon  the  spacing  and  height  of  the  conductors.  The 
characteristic  residual  voltages  of  unsymmetrical  horizontal  circuits  are 
about  20  per  cent  greater  than  those  of  symmetrical  horizontal  cir- 
cuits. They  may,  however,  be  reduced  to  those  of  the  symmetrical  cases 


FINAL    REPORT    ON     INIM'OTIVE    INTKHKKRKNCE  43 

if  the  position  of  the  intermediate  conductor  is  alternated  so  that  its 
average  position  is  midway  between  the  two  outside  conductors.  (If 
the  circuit  is  transposed  this  condition  should  he  fulfilled  in  each  section 
between  transpositions.) 

The  characteristic  residual  voltages  of  equilateral  triangular  circuits 
are  closely  proportional  to  the  conductor  spacing,  but  the  conductor  spac- 
ing has  but  little  effect  in  the  eases  of  vertical  and  horizontal  circuits. 

With  twin-circuit  lines  it  is  possible  to  interconnect  the  two  circuits 
so  that  their  unbalances  tend  to  neutralize,  giving  smaller  resultant 
unbalances  among  the  capacitances  of  pairs  of  interconnected  con- 
ductors than  the  unbalances  among  the  conductors  of  individual 
circuits.  For  twin  circuits  of  any  type  the  maximum  unbalances  occur 
when  conductors  symmetrically  located  with  respect  to  an  intermediate 
vertical  plane  are  at  common  potential.  This  arrangement  should  be 
avoided  in  all  cases. 

For  circuits  of  the  vertical  type,  or  with  top  and  lowest  conductors 
in  a  vertical  plane  and  middle  conductors  displaced  outward  a  small 
distance,  the  minimum  resultant  unbalances  are  obtained  when  the  top 
conductors  of  the  two  circuits  are  at  common  potential  and  the  middle 
and  lowest  conductors  of  one  circuit  are  at  the  potentials  of  the  lowest 
and  middle  conductors  respectively  of  the  other.  (See  Figs.  1  and  2.) 
For  triangular  and  horizontal  circuits  the  minimum  resultant  unbal- 
ances are  obtained  when  similarly  placed  conductors  of  each  circuit  are 
at  common  potential.  (See  Figs.  3,  4  and  5.)  These  figures  are  cross- 
sectional  diagrams,  the  conductors  at  common  potential  being  shown  as 
interconnected. 

The  resultant  unbalances  with  these  arrangements  are  in  some  cases 
less  than  10  per  cent  and  in  general  less  than  50  per  cent  of  those  with 
the  worst  condition  described  above.  The  arrangements  indicated  by 
Figs.  1,  2  and  3  give  resultant  unbalances  of  the  order  of  magnitude  of 
those  of  single-circuit  equilaterial  triangular  lines  of  corresponding 
conductor  spacing,  while  those  of  Figs.  4  and  5,  in  general,  give  greater 
unbalances.  In  all  cases  the  characteristic  residual  voltage  is  taken  as 
the  measure  of  the  unbalance. 

Where  ground  wires  are  used  or  in  cases  where  unsymmetrical 
circuits  or  more  than  two  circuits  are  involved,  special  study  is  neces- 
sary to  determine  the  best  arrangement. 


44 


FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 


FINAL    RKI'OUT    ON     INDI'CTIVK    1  NTKK  KKKKNi  ',]•]  45 


With  twin  circuits  of  any  configuration  if  the  interconnection 
maximum    unbalance    be    altered    by    transposing    the    interconnecting 
wires   the   unbalance    is    halved.     The   two 

possible    interconnections    resulting    from  sv^^tT«.cAL.   .^TC^CON^CCT.O^ 
this  procedure  are  shown  in  Fig.  6.     This 
plan  is  useful  when  there  is  a  doubt  as  to 
the  best  arrangement. 

To  obtain  the  greatest  advantage  of 
arrangements  giving  small  unbalances  the 
twin  circuits  should  be  interconnected  at 
both  ends  of  the  line  and  at  intermediate 
substations  where  practicable.  In  cases 
where  twin  circuits  are  paralleled  on  the 
station  side  of  transformer  banks  but  can  ris.6 

uot  be  interconnected  on  the  line  side,  it  is 

still  advantageous  to  fix  the  phase  relation  of  the  conductors  as  if  they 
were  to  be  interconnected  for  minimum   unbalances. 

When  transposing  twin-circuit  lines  to  secure  capacitance  balance, 
the  two  circuits  should  be  transposed  at  the  same  points  and  care 
should  be  taken  to  secure  the  condition  for  minimum  unbalance  in  each 
section  of  line  between  transpositions.  (See  Fig.  9,  below.) 

The  foregoing  facts  have  an  important  bearing  on  the  number  of 
transpositions  required  to  adequately  balance  different  types  of  circuits, 
more  frequent  transpositions  being  necessary  in  circuits  of  large  char- 
acteristic unbalances.  This  has  been  considered  in  IV  (c). 

EFFECT  ON  INDUCTION  FROM  BALANCED  VOLTAGES  AND  CURRENTS. 

The  type  of  power  circuit  producing  the  least  inductive  effects  in  a 
parallel  communication  circuit  depends  upon  the  spacing  of  the  con- 
ductors and  the  separation  from  the  communication  circuit.  In  general, 
for  all  typos  of  circuit,  an  increase  in  the  spacing  of  the  power 
conductors  causes  a  proportionate  increase  in  the  magnitude  of  the 
inductive  effects.  Excessive  spacing  should  therefore  be  avoided. 
On  the  other  hand,  ample  spacing  to  prevent  short-circuits  or  grounds, 
due  to  snow,  wind,  birds,  etc.,  is  essential  from  the  standpoint  of 
inductive  interference,  as  well  as  from  that  of  power  service. 

For  lines  separated  by  the  width  of  an  ordinary  highway,  a  vertical 
type  of  power  circuit,  in  general,  causes  the  smallest  inductive  effects, 
while  the  horizontal  types  cause  the  greatest  effects,  the  triangular  types 
being  intermediate  in  this  respect.  The  relative  merits  of  different 
con  figurations  vary  somewhat  with  the  separation  of  the  two  classes  of 
lines  and  with  the  dimensions  of  the  power  circuit,  depending  also 
upon  the  relative  importance  of  the  balanced  voltages  and  currents  in 
producing  induction. 

For  low-voltage  horizontal  lines,  15,000  volts  or  less,  a  symmetrical 
arrangement  of  the  conductors  is  better  than  an  unsymmetrical  arrange- 
ment. For  lines  of  any  voltage,  if  an  unsymmetrical  arrangement  is 
used,  the  intermediate  conductor  should  be  displaced  toward  the  com- 
munication circuit.  Hence,  unsymmetrical  hori/ontal  power  circuits 
along  highways  should  have  the  intermediate  conductor  placed  on  the 
side  of  the  poles  lou'artl  I  In  road,  where  communication  circuits  are,  or 
may  be,  located  on  the  opposite  side  of  the  road. 


46  FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE 

When  two  or  more  synchronous  circuits  are  carried  on  one  line  it  is 
possible  to  interconnect  the  conductors  of  the  two  circuits  or  otherwise 
fix  their  phase  relations  so  that  a  partial  neutralization  of  the  inductive 
effects  takes  place.  For  twin  circuits  of  the  vertical  type,  or  with  the 
top  and  lowest  conductors  in  a  vertical  plane  and  the  middle  conductors 

displaced  outward  a  small  distance,  the  most 
favorable  condition  is  in  general,  to  have  the 
diagonally  opposite  conductors  at  common 
potential.  (See  Figs.  7  and  8.) 

For  circuits  of  other  types  the  most  favor- 
able method  of  connection  varies  with  the 
spacing  and  height  of  the  power  conductors 
and  with  their  position  relative  to  the  com- 
munication circuit.  Thus  it  is  not  possible 
to  give  a  general  recommendation,  since 
special  study  is  required  in  each  specific  case  to  determine  the  most 
advantageous  method  of  interconnection.  Special  study  is  also  required 
for  lines  carrying  more  than  two  circuits  of  the  same  or  different  volt- 
ages, for  unsymmetrical  double-circuit  lines,  and  in  cases  where  ground 
wires  are  used. 

In  transposing  twin-circuit  lines  to  neutralize  the  inductive  effects  in 
parallel  communication  circuits,  a  similar  precaution  should  be  observed, 
as  noted  above,  with  respect  to  transpositions  for  capacitance  balance. 
(See  Fig.  9.) 

RECOMMENDED  CONFIGURATIONS. 

Taking  into  account  both  effects  above  discussed  and  practical  con- 
siderations of  construction,  the  equilateral  triangular  configuration 
(either  the  "horizontal-base"  or  "wishbone"  type)  is  in  general  recom- 
mended for  single-circuit  power  lines;  and  the  vertical  configuration 
(including  type  of  construction  with  middle  conductors  displaced 
slightly  outward  from  vertical  plane  of  the  other  two)  for  twin-circuit 
power  lines. 

The  method  of  transposing  twin  vertical  lines  to  preserve  the  best 
relation  of  interconnected  conductors  both  outside  and  inside  limits  of 
parallels  is  illustrated  in  Fig.  9,  one  barrel  being  shown  in  each  location. 

REFERENCE. 

Further  information  concerning  the  subject  discussed  in  this  exhibit 
will  be  found  in  Technical  Reports  Nos.  51,  64  and  65  of  the  Joint  Com- 
mittee on  Inductive  Interference.  These  and  other  technical  reports 
are  to  be  published  by  the  state  of  California. 

COMMENTS  ON  RULES.* 

Three  principal  features  are  to  be  noted  in  comparing  the  revised 
rules  with  General  Order  No.  39 : 

1.  The  arrangement  has  been  entirely  altered  in  order  to  group 
related  provisions  of  the  rules  and  facilitate  finding  any  particular 
subject. 

*Not  intended  to  be  included  in  proposed  General  Order. 


KKI'OKT    ON     IN'DIM'TIVK    I NTKKKKUENCE 


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48  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

2.  The  principle  of  co-operation  in  the  determination  of  means  for 
avoidance  and  mitigation  of  interference  has  been  emphasized  through- 
out in  conjunction  with  definite  guiding  rules  where  it  is  practicable  to 
introduce  them.     An  example  of  this  is  the  specific  procedure  outlined 
in  IV  (d)  for  the  transposition  of  lines  within  parallels. 

3.  Some  of  the  rules  of  a  precautionary  nature  are  not  limited  to 
lines  involved  in  parallels  and  apparatus  connected  thereto,  but  apply 
to  all  new  construction. 

Some  of  the  rules  are  discussed  in  detail  below : 

i.     General   provisions. 

(a)  Applicability  of  Rules. 

In  adopting  these  rules  in  California,  it  would  be  inadvisable,  in  this 
Committee's  opinion,  to  make  them  retroactive.  To  apply  the  rules  to 
all  existing  construction  would  call  for  extensive  changes  in  plant  on 
the  part  of  power  and  communication  companies,  particularly  in  trans- 
posing or  retransposing  power  and  telephone  lines  within  parallels. 
Considerable4  time  would  be  necessary  for  laying  out  the  transposition 
schemes,  as  each  parallel  requires  a  special  study  in  order  to  determine 
the  most  effective  scheme.  If  the  transposition  work  were  to  be  car- 
ried out  independently  of  other  reconstruction,  the  extra  cost  and 
interruption  to  service  would  be  important  items  and  would  involve  an 
undue  hardship,  except  in  cases  of  serious  inductive  interference  which 
the  rules  provide  shall  be  attended  to  promptly.  See  I  (d) .  It  has 
accordingly  been  deemed  advisable  to  limit  the  application  of  the  rules 
generally  to  new  construction,  except  those  rules  which  deal  with  main- 
tenance and  operation  where  no  reason  for  such  limitation  exists.  The 
underlying  principle  is  that  all  new  and  reconstructed  plant,  as  it  is 
built  or  installed,  shall  be  made  to  conform  with  these  rules. 

( 6 )   Co-operation. 

Without  a  proper  spirit  of  co-operation  on  the  part  of  power  and 
communication  companies,  no  rules  respecting  inductive  interference 
can  be  expected  to  have  a  full  measure  of  success.  To  get  the  best 
results,  it  is  essential  that  each  party,  operating  either  class  of  utility, 
recognize  and  have  regard  for  the  rights  and  interests  of  the  other 
party,  and  also  the  convenience  of  the  public  as  affected  by  the  several 
kinds  of  service.  An  attitude  of  indifference,  or  of  perfunctory  adher- 
ence to  the  letter  of  the  rules  without  giving  consideration  to  all  the 
various  relevant  circumstances,  may  to  a  large  extent  vitiate  the  results 
contemplated  in  these  rules.  The  right  procedure  to  employ  in  any 
case  of  interference,  actual  or  impending,  depends  upon  a  variety  of 
factors,  such  as  the  character  and  importance  of  the  service  involved, 
the  effect  on  existing  plant,  the  relation  of  present  procedure  to  future 
plans  of  the  parties  concerned,  besides,  of  course,  the  relative  costs 
and  effectiveness  of  different  procedures,  all  of  which  demand 
co-operation. 

(c)   Principle  of  Least  Cost. 

Where  there  are  two  or  more  different  methods  available  for  avoiding 
or  effectively  mitigating  an  interference,  public  interest  requires  that 
the  method  of  least  total  cost  should  be  adopted.  If  alternative  methods 


FINAL    REPORT   ON   INDUCTIVE   INTERFERENCE  49 

alVord  different  decrees  of  effectiveness,  the  method  selected,  if  not  the 
most  effective,  must  at  least  be  one  which  will  o-ivc  satisfactory  results. 
In  uvneral.  the  most  effective  method  should  be  chosen  except  where 
the  saving  in  cost  reali/ed  by  another  method  is  substantial  and  the 
sacrifice  of  effectiveness  is  not  serious.  In  applying  this  principle  to 
individual  practical  cases,  the  importance  of  the  particular  service; 
affected  should  be  taken  into  account  and  a  grade  of  service  maintained 
which  is  adequate  and  proper  for  the  conditions  of  the  case. 

(d)  Existing  Parallels. 

This  rule  has  been  drawn  to  conform  to  the  principle  explained  in 
the  discussion  under  (a)  above.  To  care  for  cases  of  serious  inter- 
ference, provision  is  made  that  such  cases  shall  be  dealt  with  promptly. 

II.     Definitions. 

(a)   Class  H  Power  Circuits. 

This  designates  a  restricted  class  of  electrical  supply  circuits  and  is 
intended  to  accord  with  the  terminology  of  other  orders  of  this  state 
and  elsewhere  relative  to  inductive  interference  and  physical  hazard. 
This  class  covers  the  great  majority  of  circuits  causing  disturbances. 
Electric  railway,  series  lighting  and  certain  other  circuits  are  treated 
separately  in  Appendix  I. 

(&)  Electrically  Connected. 

"Electrical"  connection  is  distinguished  from  "magnetic"  connec- 
tion. Electrically  connected  apparatus  affects  the  balance  of  a  circuit 
with  respect  to  earth,  while  the  corresponding  effect  of  magnetically 
connected  apparatus  is  negligible. 

(c)  Signal  Circuits. 

This  is  substantially  the  definition  given  in  the  National  Electric 
Safety  Code  (Circular  Xo.  54  of  the  U.  S.  Bureau  of  Standards). 

(d)   Communication  Circuits. 

A  restricted  class  of  signal  circuits.  Other  signal  circuits  are  con- 
sidered in  Appendix  I. 

(i)  Barrel. 

This  term,  while  most  commonly  used  in  connection  with  three-phase 
three-wire  circuits,  applies  in  principle  to  any  circuit  having  two  or 
more  metallic  conductors,  such,  for  example,  as  a  four-wire  circuit,  in 
which  ca.se  a  barrel  may  be  secured  by  three  transpositions  at  quarter 
points  in  a  uniform  section  of  line. 

(1)  and  (HI]    /><ilanc«l  ami  AY.v/V/////  Volhit/rx  ami  Ciirrt  nts. 

The  definitions  of  residuals  given  are  identical  in  meaning  with  those 
of  (Jeneral  Order  No.  Uf),  but  they  are  more  fully  explained  and  con- 
trasted with  balanced  voltages  and  currents. 


50  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

The  division  of  voltages  and  currents  of  power  circuits  into  balanced 
and  residual  components  is  fundamental  to  the  consideration  of  effects 
of  transpositions,  power-circuit  configuration,  transformer  connections, 
etc. 

III.     Location  of  lines. 

» 

(a)  Avoidance  of  Parallels. 

The  first  and  most  obvious  means  of  preventing  inductive  interference 
is  to  avoid  the  close  association  of  power  and  communication  circuits. 
Further,  it  is  recognized  that  in  no  other  way  can  complete  freedom 
from  interference  be  secured.  While,  with  the  ever-increasing  net- 
work of  electrical  circuits  of  all  kinds,  adequate  separation  to  avoid 
interference  is  becoming  increasingly  difficult  to  maintain,  the  Com- 
mittee feels  that  the  importance  of  such  separation  justifies  its  being 
made  the  first  premise  in  rules  designed  to  prevent  inductive 
interference. 

(b)  Notice  of  Intention. 

In  order  to  secure  the  time  essential  for  determining  the  proper  pro- 
cedure in  case  of  a  parallel,  advance  notice  is  necessary.  Through  the 
opportunity  for  co-operative  study  thus  afforded,  means  of  avoidance 
or  designs  to  reduce  the  interference  can  be  worked  out  in  advance  and 
applied  during  the  course  of  construction,  at  minimum  cost  and  before 
interference  develops. 

(c)  Distance  Between  Lines. 

Since  the  inductive  effects  decrease  rapidly  with  the  separation  of  the 
parallel  lines,  the  importance  of  this  rule  is  evident.  The  effectiveness 
of  transpositions  depends  upon  the  creation  of  mutually  neutralizing 
inductive  effects  in  neighboring  sections  of  circuit  and  will  in  general 
be  impaired  if  the  intensity  of  induction  is  different  in  different  sec- 
tions of  the  parallel.  Hence  uniformity  of  separation  is  important. 

(d)  Length  of  Parallel. 

Other  things  being  equal,  the  inductive  effects  of  a  parallel  increase 
in  proportion  to  its  length.  Hence,  parallels  should  be  made  as  short 
as  practicable. 

(e)  Discontinuities. 

In  general,  the  more  discontinuities  there  are  in  a  parallel  the  more 
transpositions  are  required  in  both  power  and  communication  circuits 
to  provide  effective  balance,  and  since  frequently  it  is  difficult  or 
impracticable,  in  the  design  and  layout  of  transposition  schemes,  to  take 
into  account  minor  discontinuities  which  tend  to  nullify  the  effective- 
ness of  the  transpositions,  all  discontinuities  should  be  avoided  as  far 
as  practicable  in  the  location  and  construction  of  lines. 

Many  instances  have  occurred  where  the  first  line  along  a  road  crossed 
at  frequent  intervals  in  order  to  cheapen  construction.  This  procedure 
creates  difficulties  for  any  line  subsequently  constructed,  besides  greatly 
complicating  the  transposition  scheme  required  to  reduce  inductive 
interference. 


FINAL   REPORT   ON   INDUCTIVE    INTERFERENCE  51 

IV.     Design  and  construction  of  lines. 

(a)  (rtinral  Requirements. 

As  a  primary  consideration,  power  ;md  communication  lines  which 
are  liable  to  become  involved  in  parallels  should  be  designed  and  con- 
structed in  a  proper  manner,  with  due  regard  to  features  affecting 
interference,  such  as  insulation.  Severe  transient  disturbances  to  com- 
munication circuits  due  to  abnormal  conditions  on  power  lines 
constitute  a  serious  problem  because  there  are  no  effective  means  of 
overcoming  them.  Hence,  the  importance  of  good  construction  in  pre- 
venting the  occurrence  of  failures  on  power  circuits.  Fortunately, 
construction  which  meets  this  important  need  will  also  best  insure 
uninterrupted  service  of  the  power  syslem  itself. 

(b)   Arr<tit</<  ni<  nt,  and  Spacing  of  Power  Conductors. 

The  arrangement  and  spacing  of  the  conductors  of  power  circuits  are 
of  importance  in  determining  the  circuit  unbalances  which  give  rise  to 
residuals,  and  in  determining  the  intensity  of  the  induction  in  parallel 
communication  circuits  due  to  balanced  voltages  and  currents.  By 
proper  consideration  of  this  matter  in  the  design  and  construction  of 
lines,  the  interference-producing  characteristics  of  such  lines  may  be 
materially  lessened  with  little  or  no  additional  cost.  This  is  particu- 
larly true  of  multi-circuit  lines  in  which  case  great  benefit  may  be 
secured  simply  by  care  in  fixing  the  phase  relations  among  the  conduc- 
tors of  the  different  circuits.  Due  to  lack  of  generally  available 
information  on  this  subject  it  is  deemed  advisable  to  give,  in  the  form 
of  an  exhibit  a  brief  resume  of  the  Committee's  technical  data  bearing 
on  the  subject  thus  making  possible  intelligent  compliance  with  the  rule. 

This  rule  is  not  mandatory  with  respect  to  any  particular  arrange- 
ment of  conductors,  for  while  some  arrangements  have  important 
advantages  over  others  it  is  inadvisable  to  thus  restrict  the  type  of  line 
construction,  when  in  many  cases,  by  careful  transposition  of  the  line, 
the  desired  end  can  be  attained  more  cheaply.  Wherever  practicable, 
however,  the  benefits  derived  by  favorable  arrangements  of  conductors 
should  be  sought  as  an  additional  safeguard  against  interference, 
particularly  in  case  of  major  parallels.  The  great  benefits  obtainable  by 
advantageous  methods  of  interconnection  of  twin  circuits  should  be 
secured  in  all  such  lines. 

Two  wire  branches,  metallically  connected  to  a  three-phase  circuit, 
inherently  unbalance  the  three-phase  circuits.  Even  short  branches  of 
this  character  may  cause  an  unbalance  equal  to  that  of  the  three-phase 
circuit  without  transpositions.  The  Committee  realizes  that  the  use  of 
such  branches  offers  an  economical  means  of  supplying  small  loads  and 
that  therefore  they  should  not  be  absolutely  prohibited.  Every  effort 
should,  however,  be  made  to  avoid  them. 

Single-wire  grounded  power  circuits  are  inherently  completely  unbal- 
anced, that  is,  the  entire  voltage  and  current  are  residual,  hence  they  ,-ire 
particularly  troublesome  to  parallel  communication  circuits. 


52  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

( c )   Transpositions — General. 

The  unbalanced  capacitances  to  ground  of  power  circuits  are  an 
important  cause  of  residual  voltages  and  currents.  This  is  true  of 
grounded  neutral  as  well  as  isolated  circuits  but  is  of  greater  impor- 
tance with  reference  to  the  latter.  While  such  unbalances  may  be 
lessened  by  observing  certain  precautions  in  the  arrangement  and 
spacing  of  the  conductors  as  discussed  above,  this  is  not  in  itself  suffi- 
cient and  the  circuit  must,  in  general,  be  transposed  so  that  each 
conductor  occupies  all  of  the  conductor  positions  for  equal  distances, 
with  due  regard  to  discontinuities.  In  other  words,  an  integral  number 
of  barrels,  must  be  installed  between  terminals  or  switching  points. 
The  lengths  of  barrel  specified  are  based  on  the  unbalanced  capacitances 
characteristic  of  the  different  configurations  and  upon  phase-change  and 
attenuation  at  the  harmonic  frequencies  which  cause  the  greater  por- 
tion of  the  interference  to  telephone  circuits. 

In  respect  to  metallic  communication  circuits,  particularly  telephone 
circuits,  the  more  perfectly  the  capacitances  of  the  conductors  are 
balanced,  the  less  the  disturbing  effects  of  induction  from  parallel  power 
circuits.  Unless  the  conductors  of  a  circuit  are  symmetrically  placed 
with  respect  to  earth  and  all  other  conductors  on  the  line,  transposi- 
tions are  necessary  to  equalize  the  capacitances  to  ground  of  the  two 
sides  of  the  circuit.  Telephone  circuits  are  usually  transposed  with 
respect  to  one  another  and  these  transpositions  substantially  meet  the 
requirements  of  this  rule. 

Obviously,  to  undertake  the  transposition  of  the  whole  of  an  extensive 
power  network  at  the  time  of  the  creation  of  a  parallel,  is  very 
expensive,  particularly  in  view  of  the  interruption  to  service  occasioned 
thereby.  On  the  other  hand,  in  the  initial  construction  or  during 
general  reconstruction,  an  adequate  transposition  system  can  usually  be 
installed  at  small  cost  as  a  definite  part  of  the  construction  work.  Such 
transpositions,  besides  serving  to  limit  the  residuals  in  all  parallels  with 
commercial  communication  circuits,  are  also  beneficial  in  reducing  the 
induction  in  private  telephone  circuits  which  are  often  on  the  same 
poles.  It  is  recognized  that  some  power  circuits,  particularly  those  of 
extra  high  voltage,  are  located  on  private  rights  of  way,  at  such 
distances  from  highways  that  transpositions  are  not  necessary  for  the 
protection  of  communication  lines,  and  provision  has  accordingly  been 
made  for  the  exemption  of  such  circuits  from  the  general  requirement 
of  transpositions  at  the  time  of  construction.  In  some  cases,  the 
proximity  without  transpositions  permitted  by  the  exception  may  result 
in  serious  interference,  and  special  study  should  be  given  to  cases 
which  are  on  the  border  line. 

In  the  matter  of  existing  power  circuits,  and  new  power  circuits 
coming  under  the  above  exception,  which  hereafter  become  involved  in 
parallels,  provision  is  made  that  they  shall  then  be  transposed  suffi- 
ciently to  limit  their  residual  voltages  and  currents  to  amounts  which 
can  be  tolerated.  Since  in  these  cases  two  or  more  parties  are  concerned, 
it  is  provided  that  the  necessary  transposition  system  shall  be  deter- 
mined co-operatively. 


FINAL    KKPOKT    ON     I  N  I  >l '( 'TIV  U    I  \  TKK'KKRKNCE  53 

(f/)    Tr<ins}H>xi/ions — Inside  Limits  of  Parallels. 

Transposition  systems  in  both  power  and  communication  circuits 
when  properly  co-ordinated  offe?-  the  most  reliable  and  effective  means 
of  preventing  interference  from  the  balanced  voltages  and  currents  of 
the  power  circuits. 

Tn  contrast  with  the  corresponding  rule  of  General  Order  No.  39, 
which  allowed  the  communication  company  to  specify  the  number  and 
location  of  the  transpositions  in  the  power  circuit,  this  revised  rule 
provides  that  both  parties  shall  co-operate  to  decide  upon  the  trans- 
position scheme  to  be  employed.  In  the  past  most  of  the  work  of 
designing  transposition  schemes  for  parallels  has  been  done  by  the 
communication  companies,  but  so  far  as  this  Committee  is  aware  the 
negotiations  relating  thereto  have  been  generally  conducted  in  a  spirit 
of  co-operation. 

Three-mile  barrels  in  three-phase  power  circuits  co-ordinate  satisfac- 
torily with  telephone  transposition  systems  now  available  (designed 
particularly  for  circuits  involved  in  parallels)  and  are  short  enough 
so  that  the  effect  of  phase-change*  along  the  line,  in  impairing  the 
efficiency  of  the  transposition  scheme,  is  small.  The  omission  of 
transpositions  at  the  junction  points  of  barrels  does  not  usually  impair 
the  co-ordination  of  the  transposition  systems  and  is  advantageous  in 
reducing  the  effect  of  phase-change  besides  reducing  the  number  of 
power  transpositions.  It  is  impossible  in  a  rule  to  specify  exactly  the 
spacings  of  transpositions  within  parallels  either  in  power  or  telephone 
lines  since  the  spaeings  required  vary  with  the  circumstances  being 
considerably  influenced  by  the  length  of  the  parallel  and  by  the  dis- 
continuities in  both  classes  of  line. 

Barrels  less  than  three  miles  in  length  are  often  very  useful  for  secur- 
ing economical  and  satisfactory  schemes  of  transpositions,  but  in  the 
higher  voltage  lines  the  difficulty  and  expense  of  installation  are  such 
that  they  are  justified  only  in  exceptional  cases.  For  lines  of  lower 
voltages,  parallels  are  more  numerous  and  shorter  barrels  are  practi- 
cable, also  the  greater  liability  of  discontinuities  (such  as  branch 
loads)  in  low-voltage  lines  renders  necessary  the  greater  flexibility  of 
shorter  barrels.  The  rules  accordingly  make  distinction  between  lines 
above  50,000  volts  and  those  below  50,000  volts  in  specifying  minimum 
lengths  of  barrel.  The  limitations  are  identical  with  those  of  General 
Order  No.  39. 

Several  schemes  of  co-ordinated  transpositions  can  be  designed  for 
any  given  parallel  depending  upon  the  utilization  of  existing  trans- 
positions, length  of  barrel,  type  of  telephone  transposition  system  and 
discontinuities.  Obviously,  that  scheme  involving  minimum  total  cost 
.should  usually  be  chosen.  Generally  in  a  new  parallel  it  is  economical 
to  transpose  the  new  line  to  co-ordinate  in  part,  at  least,  with  the  exist- 
ing transpositions  of  the  prior  line. 

V.      Design,  construction  and  arrangement  of  apparatus. 

(a)   Qiuilih/  and  Suitability. 

The  same  considerations  that  are  mentioned  under  IV  (a)  which 
demand  care  in  the  design  and  construction  of  power  and  communica- 

*The  effect  of  phase-change  is  discussed  in  Technical  Report  No.   66. 


54  FINAL   REPORT   ON"   INDUCTIVE   INTERFERENCE 

tion  lines  to  prevent  inductive  interference  and  to  secure  continuity  of 
service,  apply  also  to  the  apparatus  connected  to  such  lines. 

(&)   Rotating  Machinery. 

The  elimination  of  higher  harmonics  from  the  wave  form  of  rotating 
machinery  strikes  at  the  source  of  disturbance  to  telephone  circuits  by 
removing  an  underlying  cause.  It  is  obviously  a  matter  which  can  best 
be  cared  for  in  specifications  for  new  machinery. 

In  providing  that  the  deviation  from  a  pure  sine  wave  shall  not 
exceed  the  limit  set  forth  in  the  present  standardization  rules  of  the 
American  Institute  of  Electrical  Engineers,  it  is  recognized  that  this 
limit  is  unsatisfactory.  This  Committee  has'  been  in  correspondence 
with  a  committee  of  the  American  Institute  of  Electrical  Engineers 
which  has  under  consideration  the  revision  of  the  Institute's  present 
rule  on  the  subject.  It  is  expected  that  the  revised  standardization  rule 
will  be  more  satisfactory,  and  it  is  recommended  that  this  revised  rule, 
when  issued,  be  recognized  by  the  Commission. 

The  improvement  of  wave-form  in  rotating  machinery  is  essentially 
a  problem  for  the  manufacturer  and  all  new  machines  should  be 
designed  with  this  in  view.  This  Committee  understands  that  sub- 
stantial improvement  in  this  direction  can  be  effected  at  relatively 
small  cost,  hence  this  is  one  of  the  general  precautionary  rules  made 
applicable  to  all  new  construction.  For  the  improvement  of  conditions 
with  respect  to  machines  already  manufactured  and  installed,  the  use 
of  devices  or  "networks"  external  to  the  machine,  designed  to  shunt 
the  troublesome  high-frequency  components  from  the  line,  probably 
offers  the  most  economical  solution.  The  use  of  such  devices  may  even 
prove  economical  with  new  machines. 

The  provision  against  grounding  armature  windings  is  designed  to 
avoid  a  source  of  large  harmonic  residuals. 

(c)   Transformer^  and  Their  Connections. 

If  high  magnetic  densities  be  employed  in  transformers,  the  exciting 
Current  is  large,  and  large  higher  harmonic  components  are  introduced. 
The  10  per  cent  limit  on  exciting  current  here  provided  serves  to  guard 
against  extreme  designs  but  does  not  restrict  present  standard  practice. 
It  is  sometimes  economical  to  design  small  units  for  a  greater  exciting 
current.  Banks  of  such  small  units,  operated  without  neutral  ground 
connections  on  the  line  side,  do  relatively  little  harm,  hence  the 
exception.  Low  exciting  current  is  much  more  important  for  trans- 
formers of  grounded  neutral  banks  than  for  transformers  operated 
without  metallic  connection  to  ground. 

Core-type  three-phase  transformers  tend  to  suppress  their  triple- 
harmonic  exciting  currents  by  mutual  interaction  in  the  cores.  This 
is  an  important  advantage  over  shell-type  transformers,  if  there  is  a 
grounded  neutral,  as  it  helps  to  suppress  triple  harmonic  residuals. 

The  grounding  of  transformer  banks  at  such  points  of  their  windings 
as  to  unbalance  power  circuits  involved  in  parallels  is  prohibited  on 
account  of  the  residuals  thereby  caused. 

Star-connected  transformers  inherently  tend  to  cause  triple- 
harmonic  residuals  in  connected  transmission  lines  if  the  neutral  be 
grounded.  These  residuals  can  be  greatly  reduced  by  providing  a 


FIN7 AT.    REPORT   ON    INDUCTIVE   INTERFERENCE  55 

shunt  path  for  the  triple-harmonic  currents  in  the  form  of  del  la- 
connected  windings,  but  such  delta  windings  must  be  of  relatively  low 
impedance  in  order  to  be  effective.  Other  means,  viz,  operation  at 
low  magnetic  density,  interconnected-star  arrangements,  or  provision 
of  star-delta  connected  transformers  as  shunts  for  the  triple  harmonics 
of  star-star  banks,  may  prove  adequate  in  some  cases. 

Dissimilarities  of  electrical  characteristics  or  connections  among  the 
different  transformers  of  a  grounded-neutral  bank  cause  the  power 
circuit  to  be  unbalanced. 

Open-delta  connections  cause  triple-harmonic  currents  in  transmission 
lines  whereas  in  closed-delta  transformer  banks,  such  currents  are 
locally  confined. 

The  triple-harmonic  residuals,  which  occur  in  grounded  star-connected 
systems,  practically  disappear  when  the  neutrals  are  isolated.  How- 
ever, it  is  not  always  desirable  to  isolate  all  neutrals.  By  reduction  in 
operating  voltage  or  by  changes  of  transformer  taps  which  reduce  the 
magnetic  density  of  connected  transformers  relatively  large  reduction 
in  triple-harmonic  residuals  may  be  effected.  This  may  in  some  cases 
prove  the  simplest  remedy  for  excessive  residuals. 

(e)  Switches. 

The  operation  of  switches  is  sometimes  the  cause  of  severe  transient 
disturbances  in  parallel  communication  circuits.  This  is  due  in  part 
to  nonsimultaneous  operation  of  the  several  poles  of  the  switch  which 
gives  rise  momentarily  to  large  residuals.  Oil-break  switches  usually 
complete  the  opening  or  closing  of  a  circuit  within  a  few  cycles  while  air 
switches  require  a  much  longer  time. 

In  order  to  prevent  the  continuance  of  abnormal  conditions  on  power 
circuits  with  the  consequent  disturbances,  lines  involved  in  parallels 
should  be  disconnected  immediately  upon  the  occurrence  of  failures, 
and  accordingly  the  rule  provides  that  there  shall  be  at  least  one 
automatic  switch  if  prompt  manual  control  be  not  available. 

The  transient  disturbances  are  less  severe  when  switching  is  per- 
formed on  the  station  side  of  transformer  banks,  hence  this  procedure  is 
recommended  when  practicable. 

(/)  Fuses. 

The  same  consideration  of  avoiding  the  large  residuals  incident  to 
nonsimultaneous  opening  of  the  several  phases  of  a  power  circuit 
(referred  to  under  "Switches"),  dictates  the  preference  for  switches 
instead  of  fuses. 

(g)  Electrolytic  Lightning  Arresters. 

Electrolytic  lightning  arresters  not  equipped  a?  specified  in  this  rule 
ean.se  harmonic  residuals,  due  to  arcing,  which  may  result  in  severe 
disturbances  in  parallel  communication  circuits.  The  use  of  charging 
contacts  and  auxiliary  charging  resistances  largely  prevents  the  arcing 
and  lessens  the  rush  of  current  so  that  such  disturbances  are  greatly 
reduced. 


56  FINAL    REPORT   ON    INDUCTIVE   INTERFERENCE 

(h)  Special  Instruments. 

The  object  of  this  rule  is  to  provide  for  immediate  relief  from  the 
abnormal  disturbances  occasioned  by  grounds,  open  circuits,  etc.,  which 
cau.se  large  residuals  on  power  circuits,  by  giving  notice  of  the  existence 
of  such  unbalanced  conditions. 

Ammeters  in  the  neutrals  of  transformer  banks  serve  to  indicate  the 
degree  of  balance  of  the  power  circuit.  By  observing  such  meters, 
unbalances  smaller  than  those  which  would  operate  circuit  breakers 
can  be  detected.  It  is  not  necessary  that  such  ammeters  be  constantly 
in  circuit  and  suitable  provision  should  be  made  for  their  protection 
under  abnormal  conditions. 

(i)   Communication  Apparatus. 

It  is  necessary  to  balance  as  accurately  as  practicable  metallic  com- 
munication circuits  and  electrically  connected  apparatus  in  order  to 
minimize  the  disturbing  effects  of  parallel  power  circuits.  This 
requires  that  such  apparatus  be  so  designed  and  connected  as  not  to 
introduce  irregularities  either  in  the  series  impedances  of  the  two  sides 
of  the  circuit  or  in  their  admittances  to  ground.  This  applies  to  all 
such  apparatus,  including  that  used  for  superposed  grounded  signalling 
systems  (such  as  telegraph  on  telephone),  although  such  balancing  has 
no  effect  on  interference  with  the  superposed  grounded  system  itself. 

VI.     Operation   and   maintenance. 

(6)  Balance. 

It  is  to  be  expected  that  by  careful  inspection  and  maintenance 
incipient  causes  of  failures  can  be  detected  and  corrected  before  they 
develop  serious  consequences.  In  this  way  interruption  to  service  can 
be  avoided  as  well  as  severe  disturbances  to  parallel  communication 
circuits. 

(d)   Transformers. 

This  rule  is  designed  to  limit  the  introduction  of  harmonics  due  to 
excessive  magnetization  of  transformers,  which  occur  when  they  are 
operated  at  voltages  above  normal.  Such  overvoltage  operation  is 
particularly  undesirable  in  the  case  of  grounded-neutral  banks  of 
transformers. 

(e)   Switching. 

Examples  of  compliance  with  this  rule  are,  the  use  of  oil-break 
switches  with  poles  mechanically  interconnected  for  simultaneous 
action  instead  of  air-break  switches,  switching  on  station  side  of 
transformer  banks,  and  exercise  of  special  care  in  synchronizing. 

(/)   Charging  Electrolytic  Lightning  Arresters. 

In  cases  where  noticeable  interference  is  occasioned  by  charging 
electrolytic  lightning  arresters  equipped  as  required  in  V  (g),  the 
disturbance  can  be  made  less  troublesome  by  charging  them  during  the 
early  morning  hours  when  telephone  circuits  are  little  used. 


FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE  57 

(g)   Abnormal  Conditions. 

In  General  Order  No.  39,  a  more  specific  procedure  is  outlined  than 
is  here  given.  While  the  general  intent  of  the  present  rule  is  the  same, 
more  latitude  is  given,  with  the  requirement  that  operating  rules  shall 
be  developed  with  a  view  of  minimizing  the  disturbing  effects  upon 
parallel  communication  circuits. 

It  is  recognized  that  cases  will  arise  where  continuance  of  service 
over  a  faulty  power  circuit  may  be  of  greater  importance  than  the 
interruption  to  communication  service  occasioned  thereby  and  exception 
is  made  accordingly  to  the  general  rule  that  faulty  power  circuits  shall 
not  be  re-energized  for  operation  until  the  fault  is  remedied.  It  is,  of 
course,  contemplated  in  such  cases,  that  the  fault  shall  be  remedied  at 
the  first  opportunity.  . 

The  necessity  is  evident  for  having  accurate  records  of  abnormal 
occurrences  and  the  switching  incident  thereto,  for  use  in  investigations 
to  remedy  such  conditions. 

Exhibit. 

It  is  provided  in  rule  IV  (&)  that  in  the  design  and  construction  of 
Class  H  power  lines,  consideration  shall  be  given  to  the  configuration 
of  such  lines  with  a  view  to  minimizing  their  unbalances  and  their 
inductive  effects  in  neighboring  communication  circuits.  Configura- 
tions differ  widely  with  respect  to  their  characteristic  unbalances  and 
intensities  of  induction,  some  arrangements,  particularly  of  twin  cir- 
cuits, giving  only  10  per  cent  or  less  of  the  unbalances  or  induction 
given  by  others.  In  many  cases,  at  little  or  no  additional  cost, 
interference  may  be  materially  lessened  from  what  it  otherwise  would 
be  by  giving  proper  consideration  to  this  matter. 

As  such  information  is  not  always  easily  available,  this  exhibit  has 
been  prepared  to  give  a  brief  summary  of  the  Joint  Committee's 
technical  data  bearing  on  the  subject.  Doubtless  many  cases  will  arise 
which  will  be  beyond  the  scope  of  the  recommendations  made  in  this 
Exhibit.  In  such  cases  a  special  study  may  be  made.  Methods  for 
carrying  on  such  studies  are  described  in  the  Committee's  Technical 
Imports  which  are  referred  to  in  the  Exhibit. 


58  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 


APPENDIX  I. 

INTERFERENCE  NOT  COVERED  BY  RECOMMENDED  RULES. 

EXPLANATORY  NOTE.- — In  the  roles  recommended,  the  term  "communication  cir- 
cuit" is  used  in  a  restricted  sense  as  defined  in  II  (d)  and  the  power  circuits  to 
which  the  rules  apply  are  limited  to  those  defined  in  II  (a)  as  Class  H.  While 
these  include  the  circuits  most  commonly  involved  in  inductive  interference,  cases 
sometimes  occur  where  either  the  power  circuit  causing  the  induction  or  the  signal 
circuit  affected  by  induction  is  of  a  type  or  character  excluded  under  the  above 
definitions.  The  Committee  has  not  had  an  opportunity  to  make  a  special  study  of 
such  cases.  In  the  following-  the  attempt  is  made  to  state  the  governing-  principles 
involved,  for  the  assistance  of  the  Commission  in  considering  cases  of  this  character. 

(a)   Alternating  Current  Railways. 

Alternating  current  railway  trolley  circuits  as  now  generally 
operated  differ  radically  from  other  types  of  alternating  current  power 
circuits  in  that  one  side  of  the  former  is  grounded  throughout  so  that 
they  are  inherently  unbalanced,  and  moreover,  cannot  be  transposed. 
To  such  circuits  the  provisions  of  the  foregoing  rules  in  general  do  not 
npply,  and  are  not  so  intended. 

Where  railway  circuits  of  this  character  are  operated,  it  is  necessary 
to  employ  special  measures  in  order  to  prevent  inductive  interference 
with  neighboring  communication  circuits.  Other  than  separating  the 
two  classes  of  lines  where  this  is  practicable,  the  most  important  of  such 
measures  can  be  embodied  in  the  railway  construction  and  should  be 
included  in  the  design  of  the  electrification  after  a  comprehensive  study 
of  the  requirements  of  the  particular  case  by  the  parties  concerned. 
Also,  the  communication  circuits,  if  metallic,  should  be  properly  trans- 
posed and  otherwise  balanced  as  closely  as  practicable.  The  parties 
should  endeavor  to  agree  as  to  the  responsibilities  involved  and  as  to 
further  measures  to  be  adopted,  if  any  such  are  necessary.  In  the  event 
of  failure  so  to  agree  the  matter  should  be  referred  to  this  Commission. 

NOTE. — This  Committee  has  undertaken  no  investigation  of  cases  of  interference 
due  to  alternating-current  railways,  but  as  the  seriousness  of  the  inductive  effects  of 
such  railways  is  recognized,  provision  is  made  for  co-operation  when  such  cases  arise. 

(6)   Constant-Current  Lighting  Circuits. 

Care  should  be  taken  in  the  location,  design,  construction,  mainte- 
nance and  operation  of  constant-current  lighting  circuits  (both  direct- 
current  and  alternating-current)  to  avoid,  so  far  as  practicable,  inductive 
interference  with  communication  circuits.  In  particular  every 
reasonable  effort  should  be  made  to  avoid  creating  new  conditions  which 
would  produce  such  interference,  especially  where  interexchange 
telephone  lines  are  affected.  In  cases  where  such  conditions  are 
unavoidable,  remedial  measures  should  be  employed  as  may  be  necessary, 


FINAL  REPORT  ON  INDUCTIVE  INTERFERENCE  59 

the  details  of  which  should  bo  agreed  upon  by  the  parties  concerned  in 
general  accordance  with  the  following  provisions: 

1.  Where  necessary,  the  two  sides  of  the  lighting  circuit  should  be 

run  on  one  pole  line  within  the  section  where  the  interference 
is  set  up  and  co-ordinated  systems  of  transpositions  applied 
to  the  lighting  and  telephone  circuits. 

2.  Preference  should  be  given  to  those  types  of  lamps  and  other 

equipment  which  do  not  introduce  high  frequency  components 
in  the  lighting  current.  The  use  of  incandescent  lamps 
instead  of  arc  lamps  is  usually  advantageous  in  this  respect. 
X.  Due  regard  should  be  given  to  the  insulation  and  balance  of  both 
1lic  lighting  and  communication  circuits.  Balance  of  the 
lighting  circuit  requires  equalization  of  the  voltages  to  ground 
of  the  two  sides  of  the  circuit  within  the  section  where  the 
two  circuits  are  in  proximity.  This  necessitates  that  the  cir- 
cuit be  well  insulated  and  in  general  that  the  lamps  be  similarly 
distributed  in  the  two  sides  of  the  circuit  with  equal  numbers 
of  lamps  in  the  two  sides  between  the  source  of  supply  and 
the  section  of  proximity. 

NOTE. — It  is  common  practice  in  city  lighting  to  run  single-wire  circuits  through 
many  lamps  in  series,  scattered  widely,  instead  of  carrying  the  return  conductor  on 
the  same  line,  or  win-re  the  two  conductors  are  on  the"  same  line,  without  balancing 
their  voltnges  to  ground.  Both  of  these  features  tend  to  create  residuals  and  to 
cause  severe  inductive  effects  in  neighboring  signal  circuits. 

It  should  be  practicable,  by  care  in  laying  out  such  lighting  circuits,  and  in  locating 
important  telephone  lines,  such  as  toll  lines  which  occupy  but  a  few  streets,  to  avoid 
close  proximity  between  these  classes  of  circuits.  In  those  cases  where  proximity  is 
unavoidable,  it  is  possible,  by  running  both  sides  of  the  lighting  circuit  close  jtogether 
on  the  same  line,  by  care  in  distributing  the  lamps  and  by  transposing  the  circuit 
within  the  section  of  proximity,  greatly  to  reduce  the  residuals. 

A  considerable  difference  exists  among  the  various  types  of  lamps  used  in  that  arc 
lamps  introduce  large  harmonics  into  the  lighting  circuit,  while  incandescent  lamps 
produce  no  appreciable  distortion. 

The  balancing  of  a  lighting  circuit  can  be  accomplished  in  many  different  ways 
depending  upon  the  specific  conditions.  The  simplest  general  procedure  is  outlined 
above. 

(c)  Power  Circuits  of  Lower  Voltages. 

In  case  of  interference  with  the  operation  of  communication  circuits 
by  constant  potential  alternating-current  circuits  of  voltage  lower  than 
the  limits  specified  in  the  definition  .of  a  Class  H  power  circuit,  the 
parties  concerned  should  agree  upon  remedial  measures  in  general 
accordance  with  the  rules  recommended  in  this  report  and  should 
co-operate  in  applying  such  measures  to  the  extent  that  may  be  neces- 
sary, as  follows: 

1.  Where  practicable  at  reasonable  expense,  the  lines  should  be, 

separated  sufficiently  to  avoid  interference. 

2.  Co-ordinated  systems  of  transpositions  should  be  applied  to  both 

classes  of  circuits  within  the  section  where  the  interference  is 
set  up. 


60  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

3.  If  practicable,  the  residual  voltage  and  current  of  the  power 

circuit  should  be  reduced. 

4.  Due  regard  should  be  given  to  the  insulation  and  balance  of 

metallic  communication  circuits. 

5.  Consideration   should  be   given  to   the  reduction  of  the  high- 

frequency  components  of  the  voltages  and  currents  of  the 
power  circuit. 

NOTE. — The  physical  principles  upon  which  the  rules  as  a  whole  are  based  apply 
in  ease  of  power  circuits  of  all  voltages,  the  differences  being  only  quantitative  in 
respect  to  the  relative  importance  of  different  factors. 

(d)   Cables. 

In  case  of  inductive  interference  where  either  the  power  circuit  or 
the  communication  circuit  is  carried  in  cable,  consideration  should  be 
given  to  the  employment  of  such  remedial  measures,  included  in  the 
rules  recommended  or  otherwise  as  may  be  reasonably  applicable. 

In  such  cases,  particular  features  to  which  attention  should  be 
directed,  are:  (1)  limiting  the  residual  current  of  the  power  circuit, 
(2)  balancing  the  communication  circuits  if  they  are  metallic,  and  (3) 
transposing  the  communication  circuits,  if  they,  are  metallic  and  in 
open  wire. 

NOTE. — Where  cables  are  used  for  either  power  or  communication  circuits  within 
sections  where  these  two  clases  of  circuits  are  in  proximity,  there  is,  in  general,  far 
less  liability  of  interference,  and  many  provisions  of  the  recommended  rules  are 
inapplicable.  In  some  cases,  however,  residual  currents  in  cabled  power  circuits  may 
cause  interference  to  either  open-wire  or  cabled  communication  circuits,  and  open- 
wire  power  circuits  sometimes  cause  severe  disturbance  to  communication  circuits 
which  are  in  cable. 

(e)   Telephone  Subscribers'  Circuits. 

In  case  of  inductive  interference  by  any  electric  power  circuit  with 
metallic  telephone  subscribers'  circuits  (which  will  usually  occur  only 
when  the  latter  are  long  open-wire  surburban  loops)  the  parties  con- 
cerned should  agree  upon  a  plan  for  avoiding  the  interference  by 
removal  of  one  of  the  lines  or  for  mitigating  the  interference  by 
remedial  measures,  as  the  circumstances  may  require,  in  general 
accordance  with  the  recommended  rules. 

NOTE. — Telephone  circuits  falling  within  this  class  are  far  more  numerous  than 
those  included  under  the  definition  of  communication  circuit — II  (d) .  Fortunately, 
subscribers'  telephone  circuits  are  not  as  a  rule  seriously  exposed  to  the  influence  of 
power  circuits  since  they  are  generally  short  and  are  often  run  partly  in  cable.  When, 
however,  they  are  exposed,  the  disturbing  effects  are  relatively  more  severe  on  account 
of  the  closer  proximity  of  the  parallel  to  the  subscriber's  station.  A  parallel  involv- 
ing a  subscriber's  circuit  does  not  affect  so  large  a  portion  of  the  public  as  a  parallel 
involving  an  interexchange  circuit  .and  on  such  a  basis  the  former  is  of  less  impor- 
tance. However,  this  does  not  justify  excluding  such  circuits  from  all  protection 
against  interference.  Consequently,  with  due  regard  to  the  relative  importance  of  the 
service  affected,  telephone  subscribers'  circuits  should  be  afforded  such  protection  as 
is  necessary  in  general  accordance  with  the  principles  governing  the  protection  of 
other  communication  circuits. 


FINAL    RKI'ORT    ON"    INDUCTIVE    INTERFERENCE  61 

(/)  Direct  Current  Circuits. 

In  cases  of  inductive  interference  with  communication  circuits  due  to 
constant  potential  direct  current  circuits,  usually  occurring  only  where 
grounded  railway  trolley  circuits  and  telephone  circuits  are  in 
proximity,  adequate  remedial  measures  should  be  agreed  upon  and  put 
into  effect  by  the  parties  concerned.  Where  telephone  circuits  are 
involved,  in  addition  to  transposing  and  balancing  such  circuits,  special 
consideration  should  be  given:  (1)  to  securing  generators  and  motors 
having  a  voltage  as  free  as  practicable  from  high-frequency  waves;  and 
(2)  to  the  use  of  special  devices  external  to  the  generators,  motors  and 
rectifiers  which  tend  to  absorb  the  high-frequency  currents  and  thereby 
prevent  their  appearance  in  the  line. 

NOTE. — High-frequency  components  may  occur  in  constant-potential  direct-current 
circuits  and  occasionally  constitute  a  source  of  interference.  This  is  particularly 
true  of  electrified  railway  circuits  which  use  a  large  amount  of  power.  It  is  there- 
fore provided  that  effort  be  made  to  secure  apparatus  as  free  as  possible  from  such 
high-frequency  components  and  that  if  necessary,  suitable  shunt  paths  be  provided  to 
confine  these  high-frequency  components  to  local  circuits. 

(g)   Other  Cases  of  Interference. 

If  any  case  of  inductive  interference,  not  otherwise  covered  by  this 
report,  should  be  experienced  or  become  imminent,  the  parties  con- 
cerned should  endeavor  to  agree  upon  a  procedure  for  avoiding 
preferably,  or  if  avoidance  be  not  feasible,  for  mitigating  the  interfer- 
ence by  applying,  to  such  extent  as  may  be  necessary,  the  measures  set 
forth  in  this  report,  or  by  other  means. 


62  FINAL   REPORT    ON"   INDUCTIVE   INTERFERENCE 


APPENDIX  II. 

LIST  OF  TECHNICAL  REPORTS. 

The  following  is  a  list  of  the  technical  reports  which  have  been 
prepared  in  connection  with  the  investigation  by  the  Joint  Committee  on 
Inductive  Interference.  The  reports  recommended  for  publication  are 
indicated  by  an  asterisk  (*)  after  the  number. 

Technical 

report  Subject  or  title. 

number. 

1*  General  outline  of  tests  to  be  made  at  Salinas  on  parallels  between  lines  of  the 
Sierra  and  San  Francisco  Power  Company,  the  Western  Union  Telegraph 
Company,  the  Southern  Pacific  Company,  and  The  Pacific  Telephone  and 
Telegraph  Company.  (6  pages.)  Dated  January  6,  1913. 

2*  Summary  of  results  of  tests  at  Morgan  Hill  on  parallel  between  lines  of  the 
Coast  Counties  Gas  and  Electric  Company  and  The  Pacific  Telephone  and 
Telegraph  Company  hpjwppn  Morgan  Hill  and  Gilrov.  (8  pages.)  Dated 
February  3,  1913. 

3*  A  description  of  the  noise  standard  in  use  for  measuring  noise  on  telephone 
circuits  in  terms  of  a  standard  unit.  (3  pages,  1  drawing.)  Dated  Feb- 
urary  24,  1913. 

4*  A  description  of  the  instruments  and  methods  used  for  the  measurement  of 
effective  values  of  induced  voltages  and  currents.  (2  pages.)  Dated 
February  24,  1913. 

5*  A  description  of  apparatus  and  connections  used  in  measuring  line  and  residual 
currents  and  voltages  of  power  circuits.  (4  pages,  2  drawings.)  Dated 
February  24,  1913. 

(5  Tests  of  the  effects  of  opening  the  secondary  delta  of  the  autotransformer  bank 
at  Salinas.  (7  pages.)  Dated  March  31,  1913. 

7  Tests  of  the  induction  in  the  block  signalling  circuits  of  the  Southern  Pacific 
Company  paralleled  by  the  Salinas-King  City  circuit  of  the  Coast  Valleys 
Gas  and  Electric  Company.  (3  pages,  1  drawing.)  Dated  March  31,  1913. 

S  Tests  of  the  induction  in  the  telephone  circuits  of  exposure  No.  2  at  Salinas 
under  normal  operating  conditions  of  the  power  system  with  particular 
reference  to  the  effects  of  grounding  and  isolating  the  neutral  of  the  Salinas 
autotransformers.  (15  pages,  1  drawing.)  Dated  March  31,  1913. 

1)  Experimental  determination  of  the  coefficients  of  induction  for  residual  cur- 
rents and  voltages  in  exposure  No.  2  at  Salinas.  (4  pages.)  Dated  March 
31,  1913. 

10       Measurements  of  the  harmonics  of  the  neutral  current  at  Salinas.      (3  pages, 

1  drawing.)      Dated  March  31,  1913. 

1  1  Investigation  of  current  transformers,  ratios  and  errors  due  to  the  use  of  cur- 
rent transformers  under  the  conditions  of  the  tests.  (17  pages,  4  drawings.) 
Dated  April  7,  1913. 

12  Formula?  for  the  computation  of  electrostatic  and  electromagnetic  induction 
from  power  circuits  in  neighboring  communication  circuits.  (14  pages,  4 
drawings. )  Dated  March  31,  1913. 

13.  An  investigation  of  errors  in  measurements  of  residual  voltage  due  to  the 
potential  transformers  used  and  a  discussion  of  the  method  of  measurement 
at  Salinas.  (28  pages,  2  drawings.)  Dated  September  3,  1913. 
14  Comparative  tests  of  the  noise  in  exposed  telephone  circuits  with  power  on  and 
off  the  55,000- volt  power  circuit  of  the  Sierra  and  San  Francisco  Power  Com- 
pany between  Guadalupe  and  Salinas.  (7  pages,  1  drawing.)  Dated 
April  30,  1913. 


l-'LNATj    KKl'OKT    ON     INDUCTIVE    INTERFERENCE  <Jo 

Technical 

report  Subject  or  title. 

number. 

15  Supplementary  to  Technical  Report  No.  8,  differing  from  the  earlier  report  in 
that  the  telephone  circuits  were  shielded.  Contains  a  discussion  of  trails- 
positions.  (22  pages.)  Dated  June  3,  1913. 

1(»  Tests  of  induction  in  telephone  circuits  exposed  to  the  Coast  Counties  Gas 
and  Electric  Company's  22,000-volt  line  between  Morgan  Hill  and  Gilroy 
with  the  power  circuit  uutransposed  and  open  at  Gilroy.  (4  pages.) 
Dated  April  30,  1913. 

17  Tests  of  the  induction  in  telephone  circuits  exposed  to  the  Coast  Counties  Gas 
and  Electric  Company's  22,000-volt  line  between  Morgan  Hill  and  Gilroy, 
before  and  after  installing  power-circuit  transpositions.  (24  pages,  1  draw- 
ing.) Dated  April  30,  1913. 

18.  Tests  of  the  effect,  on  exposed  telephone  circuits,  of  grounding  one  phase  of 
the  Coast  Counties  Gas  and  Electric  Company's  22,000-volt  three-phase 
delta-connected  line.  (4  pages.)  Dated  April  30,  1913. 

It)  Tests  of  the  combined  effect  of  the  Coast  Counties  Gas  and  Electric  Company's 
and  the  Sierra  and  San  Francisco  Power  Company's  circuits  on  the  tele- 
phone circuits  in  the  exposure  between  Morgan  Hill  and  Gilroy.  (4  pages.) 
Dated  April  30,  1913. 

L'o  Tests  of  the  effect  on  the  residual  voltage  of  transposing  the  Coast  Counties 
Gas  and  Electric  Company's  22,000-volt  line  within  the  exposure  between 
Morgan  Hill  and  Gilroy.  (3  pages.)  Dated  April  30,  1913. 

21  Tests  to  determine  the  comparative  effect  on  the  noise  in  the  exposed  telephone 
circuits  of  having  the  power  on  and  off  the  Coast  Counties  Gas  and  Electric 
Company's  22,000-volt  line  between  Morgan  Hill  and  Gilroy,  and  the  effect 
of  shielding  the  telephone  circuit  under  test  by  grounding  other  circuits  on 
the  lead.  (4  pages.)  Dated  April  30,  1913. 

-''2  Computation  of  the  coefficients  of  induction  from  balanced  and  residual  currents 
and  voltages  for  the  telephone  circuits  of  exposure  No.  2  at  Salinas.  (15 
pages,  4  drawings.)  Dated  June  3,  1913. 

2.'5  Experimental  determination  of  the  coefficients  of  induction  from  residual  cur- 
rents and  voltages,  for  the  telephone  circuits  of  exposure  No.  2  at  Salinas- 
more  complete  than  Technical  Report  No.  9.  (23  pages,  1  drawing.)  Dated 
June  3,  1913. 

21  Comparison  of  computations  of  Technical  Report  No.  22  with  exiKHMinental  data 
of  Technical  Report  No.  23.  (10  pages,  6  drawings.)  Dated  September  ."•, 
1913. 

25  Tests  of  induction  in  telephone  circuits  in  exposure  between  Salinas  and  King 
City  under  normal  operating  conditions,  with  the  neutral  of  the  Salinas 
autotransformers  grounded  and  isolated.  (20  pages.)  Dated  June  3,  1.1)1  :j. 

2<J  Tests  of  accuracy  of  measurement  of  residual  current  by  certain  current 
transformers.  (3  pages,  1  drawing.)  Dated  June  3,  1913. 

27  Tests  of  induction  in  telephone  circuits  in  exposure  No.  2  at  Salinas  with  the 

North  Beach  steam  station  energizing  the  Sierra  and  San  Francisco  Power 
Company's  line.  Supplementary  to  Technical  Reports  Nos.  8  and  15,  dif- 
fering in  the  sources  of  supply  of  the  power  system.  (21  pages.)  Dated 
July  14,  1913. 

28  Supplementary  to  Technical  Reports  Nos.  8  and  15.     Voltage  lowered  5  per 

cent   at  the   Guadalupe   autotransformers   which   supply   the   power  circuit. 

(20  pages.)     Dated  September  3,  1913. 
29*     Determination  of  impedances  of  lines,  by  computations  and  by  measurements — 

numerous  curve  sheets  and  tables.     (26  pages,  39  drawings.)     Dated  May 

4,  1914. 
30       Tests  of  induction  in  telephone  circuits  in  exposure  Nos.  1  and  2  at  Salinas, 

with  the  neutral  of  the   Salinas  transformers  grounded  and  isolated.     (10 

pages. )     Dated  September  3,  1913. 


64  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

Technical 

report  Subject  or  title. 

number. 

31  Supplementary    to    Technical    Reports    Nos.    8    and    15    and    more    complete. 

Includes  tests  with  Salinas  neutral  grounded  and  isolated  and  with  telephone 
circuits  shielded  and  unshielded.  (29  pages.)  Dated  September  2,  1913. 

32  Supplementary  to  Technical  Report  No.  25.      (22  pages.)      Dated   September 

3,  1913. 

33  Induction  in  test  leads  used  at  Salinas  for  connecting  testing  apparatus  to  the 

circuits  of  exposure  No.  2,  and  the  effect  of  such  on  the  measurements  of 
the  induction  from  the  exposure.  (20  pages.)  Dated  September  3,  1913. 

34  Effect   of  changes   in   the   insulation   resistance   of  the  telephone   line  on   the 

induction  in  telephone  circuits  of  exposure  No.  2  at  Salinas.  Also  supple- 
ments Technical  Reports  Nos.  8,  15,  and  31.  (22  pages,  2  drawings.) 
Dated  September  3,  1913. 

35  General  outline  of  tests  to  be  made  at  Santa  Cruz  on  the  parallel  between  lines 

of  the  Coast  Counties  Gas  and  Electric  Company  and  The  Pacific  Telephone 
and  Telegraph  Company.  (4  pages.)  Dated  July  14,  1913. 

36  Induction  in  telegraph  circuits  of  the  Western  Union  Telegraph  Company  and 

the  Southern  Pacific  Company  in  exposure  No.  1  between  Salinas  and  Sail 
Jose.  (8  pages.)  Dated  September  3,  1913. 

37  Noise  tests  on  telephone  circuits  radiating  from  Salinas,  with  the  neutral  of 

the  Salinas  autotransformers  grounded  and  isolated.  (4  pages.)  Dated 
September  3,  1913. 

38*  General  review  of  tests  at  Salinas.  Summarizing  reports  1,  6,  7,  8,  9,  10,  11, 
12,  13,  14,  15,  22,  23,  24,  25,  26,  27,  28,  30,  31,  32,  33,  34,  36,  37.  (52 
pages,  1  drawing.)  Dated  October  13,  1913. 

39  General    consideration    of   transpositions    and    a    study    of    the    results    to    be 

expected  from  the  application  of  various  transposition  schemes  to  the  Santa 
Cruz-Watsonville  exposure.  (29  pages,  5  drawings.)  Dated  December  15, 
1913. 

40*  Method  of  measurement  of  capacitance  and  conductance  unbalances.  (1 
page,  1  drawing.)  Dated  December  15,  1913. 

41*  Harmonic  analysis  of  alternating  current  waves  by  oscillograph  and  resonant 
shunt.  Comparison  of  the  methods.  (27  pages,  3  drawings.)  Dated  March 
16,  1914. 

42*  Investigation  of  the  current  transformers  in  use  at  Santa  Cruz,  to  determine 
their  ratios  of  transformation  and  suitability  for  residual  current  measure- 
ments. (29  pages,  6  drawings.)  Dated  January  26,  1914. 

43  Outline  of  tests  to  determine   the  effect  of  extraneous  current  on   the   intel- 

ligibility of  telephone  conversation.  (7  pages,  1  drawing.)  Dated  March  16, 
1914. 

44  Induction   in   the   telephone   circuits  of  the   Santa   Cruz-Watsonville  exposure 

and  in  the  test  lead's,  from  sources  other  than  the  22,000-volt  line.  (12 
pages.)  Dated  March  16,  1914. 

45  Induction   in   the   telephone   circuits  of  the   Santa   Cruz-Watsonville  exposure 

under  commercial  operating  conditions,  with  the  original  transpositions  in 
both  power  and  telephone  lines.  (15  pages.)  Dated  March  16,  1914. 

40  Supplementary  to  Technical  Report  No.  39.     A  study  of  additional  transposi- 

tion schemes  for  the  Santa  Cruz-Watsonville  exposure.  (12  pages,  2  draw- 
ings.) Dated  May  4,  1914. 

47  Computation  of  the  coefficients  of  induction  for  balanced  and  residual  currents 

and  voltages  for  the  Santa  Cruz-Watsonville  exposures.  (13  pages,  2  draw- 
ings.) Dated  May  4,  1914. 

48  Experimental   determination    of   coefficients   of   induction   in    the   Santa   Cruz- 

Watsonville  exposure,  with  the  original  transpositions.  (42  pages.)  Dated 
May  4,  1914. 


FINAL   REPORT    ON    INDUCTIVE   INTERFERENCE  65 

Technical 

report  Subject  or  title. 

number. 

4!)  Further  experimental  determination  of  coefficients  of  induct iou  for  balanced 
voltages,  in  the  Santa  Cruz-Watsonville  exposure,  with  the  original  trans- 
positions. (13  pages.)  Dated  May  4,  1914. 

50*  Study  of  the  influence  of  various  transformer  connections  and  flux  densities  on 
the  third  harmonic  and  its  multiples  in  a  three-phase  circuit.  (IS  pages,  '2 
drawings.)  Dated  October  0,  1914. 

51*  Residual  voltage  due  to  the  line  unbalance  of  power  circuits  isolated  from 
ground.  Effect  of  circuit  configuration  transpositions  and  frequency.  (69 
pages,  38  drawings.)  Dated  September  30,  1916. 

__*  Memorandum  supplementing  Technical  Report  No.  51.  (2  pages.)  Dated 
February  28,  1917. 

__*  Memorandum  supplementing  Technical  Report  No.  51.  (2  pages,  1  drawing.* 
Dated  July  1C,  1917. 

52*  Residuals  produced  by  a  ground  on  one  phase  of  a  normally  isolated  three- 
phase  system.  (29  pa.ues.  2  drawings.)  Dated  July  28,  1915. 

__*  Memorandum  supplementing  Technical  Report  No.  52.  (4  pages.)  Dated 
July  3,  1917. 

."».'{  Investigation  of  current  transformers — San  Fernando.  (7  pages,  1  drawing.) 
Dated  July  29,  1915. 

54*  San  Fernaudo-Somis  parallel.  Experimental  determination  of  coefficients  of 
longitudinal  induction  at.  50  cycles.  Effect  of  transpositions  in  power  cir- 
cuits. (30  pages.)  Dated  October  1,  1915. 

55*  Experimental  determination  of  coefficients  of  induction  from  residuals  at 
telephonic  frequencies — Effect  of  admittance  unbalance.  San  Fernando- 
Somis  parallel.  (26  pages,  6  drawings.)  Dated  January  7,  1916. 

56*  Determination  of  coefficients  of  induction  in  a  short  section  of  the  San 
Fernando-Somis  parallel.  Measurements  and  computations.  Effect  of  tele- 
phone transpositions.  (41  pages,  6  drawings.)  Dated  February  10,  1916. 

57  Determination  of  coefficients  of  transverse  induction  from  residuals  at  tele- 
phonic frequencies  in  a  short  section  of  the  San  Fernando-Somis  parallel. 
(8  pages,  3  drawings.)  Dated  February  14,  1916. 

58*  Investigation  of  potential  transformers  for  residual  voltage  measurements. 
(42  pages,  4  drawings.)  Dated  April  14,  1916. 

51)*  Relation  of  triple  harmonic  residuals  in  a  transmission  line  to  the  magnetic 
density  in  connected  transformer  banks.  Effect  of  the  line  characteristics. 
(61  pages,  14  drawings.)  Dated  September  6,  1916. 

60*  Triple-harmonic  residuals  as  affected  by  certain  types  of  three-phase  star 
connection  of  transformers.  (24  pages,  4  drawings.)  Dated  July  26,  1916. 

61*  Measurements  of  residual  voltages  and  currents  of  several  transmission  systems. 
(29  pages,  1  drawing.)  Dated  July  29,  1916. 

62*  Double-frequency  voltages  and  currents  in  a  three-phase  transmission  line.  (13 
pages,  1  drawing.)  Dated  March  18,  1916. 

63*  Standard  forms  for  recording  data  and  computations.  (22  pages.)  Dated 
March  8,  1916. 

64*  Computation  of  induction  between  parallel  power  and  communication  circuits. 
(46  pages,  1  drawing.)  Dated  August  4,  1916. 

65*  Coefficients  of  induction  for  communication  circuits  paralleled  by  three-phase 
power  circuits.  Variation  with  relative  position  and  configuration.  (54 
pages,  214  drawings,  92  tables.)  Dated  January  6,  1917. 

(HI*  Co-ordination  of  transposition  systems  for  power  and  telephone  circuits.  (63 
pages,  19  drawings.)  Dated  January  6,  1917. 

67*  Notes  on  the  transposition  of  power  circuits,  and  private  telephone  circuits.  (9 
pa-es,  3  drawings.)  Dated  August  14,  1917. 


66  FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE 

Technical 

report  Subject  or  title. 

number. 

OS*     Effect    of    protective    ground    wires    of    power    lines    on    induction    in    parallel 

communication  circuits.      (7  pages,  1  drawing.)      Dated  June  28,  1917. 
69*     Relation   of  currents   in    terminal   apparatus  of  telegraph  circuits  to   induced 

voltages  and  location  of  parallel.      (8  pages,  2  drawings.)      Dated  July  19, 

1917. 
70*     The   relative   importance    of    the   volt-mile    (electric   induction)    and    the    volt 

(magnetic  induction)    in  causing  interference  with  telephone  circuits.      (10 

pages,  1  drawing.)     Dated  August  17,  1917. 
71*     The  influence  of  wave-form  on  the  detrimental  effect  of  induction.      (11  pages, 

1  drawing.)     Dated  August  10,  1917. 

The  following  reports,  technical  in  character,  were  prepared  at  the 
request  of  the  Joint  Committee,  but  outside  of  its  organization: 

Tests  to  determine  the  effect  of  extraneous  current  of  single  frequency  on  the 
intelligibility  of  a  telephone  conversation.  (13  pages,  14  drawings.)  Dated 
December  22,  1914.  Laboratory  investigation  by  Engineering  Department 
of  American  Telephone  and  Telegraph  Company. 

Tests  to  determine  the  effect  of  extraneous  single  frequency  current  on  tele- 
graph transmission.  (40  pages,  29  drawings.)  Dated  September  14,  1916. 
Joint  laboratory  investigation  by  engineers  of  the  Western  Union  Telegraph 
Company,  the  Postal  Telegraph-Cable  Company  and  the  American  Telephone 
and  Telegraph  Company. 

The  development  of  balance  of  telephone  circuits.  Practice  of  telephone 
companies  in  balancing  their  lines  and  apparatus  as  a  means  of  reducing 
induction  from  foreign  circuits  and  other  telephone  circuits.  (61  pages,  20 
drawings.)  Dated  June  15,  1015.  Submitted  by  A.  H.  Griswold  on  behalf 
of  the  telephone  interests. 


KIXAh    REPORT    ON    INIMVTIVE    INTERFERENCE  67 


APPENDIX  III. 

COMMENTS  ON  THE  REPORT  OF  JULY  7,  1914,  BY  THE  JOINT  COM- 
MITTEE ON  INDUCTIVE  INTERFERENCE  TO  THE  RAILROAD 
COMMISSION  OF  THE  STATE  OF  CALIFORNIA. 

The  investigations  conducted  since  the  issuance  of  the  preliminary 
report  in  1914  have  greatly  extended  the  detailed  .knowledge  of  the 
various  factors  affecting  inductive  interference,  and  have  disclosed  a 
1Vw  inisstatements  in  the  former  report.  The  new  matter  is  contained 
in  Technical  Reports  Nos.  51  .ff.f  listed  in  Appendix  II.  The  purpose  of 
this  appendix  is  to  comment  on  certain  statements  of  the  preliminary 
report  and  to  correct  misstatements. 

In  Appendix  I  of  the  former  report  in  the  next  to  the  last  paragraph 
(on  pa^-e  21)*,  investigations  are  mentioned,  to  be  conducted  by  the 
American  Telephone  and  Telegraph  Company,  Western  Union  Tele- 
graph Company  and  Postal  Telegraph-Cable  Company.  The  reports 
upon  these  investigations  have  been  received  and  are  listed  in  Appendix 
II  of  the  present  report. 

In  Appendix  II  of  the  former  report  under  "3.  Means  for  Preventing 
or  Reducing  Residual  Voltages  and  Currents" — fifth  paragraph,  line  5, 
ff  (beginning  in  the  sixth  line  from  the  bottom  of  page  28)*  it  is  stated 
that  ''With  a  horizontal  arrangement  of  conductors,  the  capacities  to 
ground  are  more  nearly  equal  than  with  the  triangular  or  vertical 
arrangement."  As  shown  in  Technical  Report  No.  51  (see  also  the 
exhibit  of  Part  III  of  this  report),  the  fact  is  that  lines  having  their 
conductors  in  a  plane,  vertical  or  horizontal,  present  the  worst  unbal- 
ances of  capacitances  to  ground  experienced  with  ordinary  configura- 
tions. Briefly,  the  triangular  configuration  is  in  general  the  best 
balanced;  vertical  and  symmetrical  horizontal  configurations  are  very 
nearly  alike  with  much  greater  unbalances  than  the  triangular,  and  a 
little  better  than  the  unsymmetrical  horizontal,  which  is  the  worst. 

With  respect  to  the  remainder  of  the  same  paragraph,  investigation 
has  shown  that  the  "electrostatic  capacities"  are  "the  controlling 
factors  in  determining  the  residual  voltage  and  current  of  an  isolated 
system  under  normal  operation"  with  proper  insulation;  and  that 
"properly  spaced  transpositions"  are  "substantially  effective,"  thus 
confirming  the  statements  previously  made. 

It  should  be  noted  that  with  a  grounded  neutral  (see  second  para- 
graph of  page  29)  a  residual  current  exists  corresponding  to  the  residual 
voltage  of  the  isolated  system,  and  is  similarly  controllable  by  transposi- 
tions. This  matter  is  discussed  in  Technical  Report  No.  51. 

In  Appendix  III  of  the  former  report,  under  "1.  Effect  of  Trans- 
positions in  Reducing  Induction,"  fourth  paragraph,  the  words  "series 
impedance"  should  be  added  before  the  word  "capacity"  in  line  10 
(the  llth  line  from  bottom  of  page  32),*  so  the  sentence  will  read 
"The  voltage  induced  along  the  conductors  of  the  telephone  circuit  and 
the  induced  voltage  to  ground  would  be  present  but  would  not  be 

*Page  references  are  to  edition  of  report  published  by  the  California  State  Printing 
Office,  1914. 


68  FINAL   REPORT   ON   INDUCTIVE   INTERFERENCE 

effective  in  producing  any  voltage  between  the  conductors  of  the  tele- 
phone circuit,  provided  the  series  impedance,  capacity  and  leakage  to 
ground  of  each  side  of  the  telephone  circuit  were  equal." 

In  the  same  Appendix,  in  the  next  to  the  last  paragraph  of  "1. 
Effect  of  Transpositions  in  Reducing  Induction"  (page  33)*  the  word 
"not"  was  omitted  after  "can."  The  first  sentence  should  read  "If 
the  communication  circuit  has  a  ground  return,  it  can  not  be  trans- 
posed and  the  power  circuit  transpositions  alone  (referring  to 
transpositions)  will  be  effective  in  reducing  interference  arising  from 
balanced  currents  and  voltages." 

With  the  foregoing  changes,  the  discussion  given  in  the  preliminary 
report  is  considered  correct,  though  much  more  complete  discussions  of 
various  of  its  topics  are  to  be  found  in  the  Technical  Reports. 

*Page  references  are  to  edition  of  report  published  by  the  California  State  Printing 
Office,  -1914. 


FIXAh    RKI'OKT    ON'    1NIMTTIVK    INTKKKKKKNCE  69 


APFENDIV  IV. 

BIBLIOGRAPHY. 

In  the  course  of  its  investigation  the  Committee  has  collected  numer- 
ous references  to  the  published  lilenitnre  of  subjects  related  to  its  work. 
A  list  of  such  references  which  deal  directly  with  inductive  interference 
is  irivrii  lielnw  fdi-  the  benefit  of  those  interested  in  further  study. 

.Many  discussions  on  related  subjects,  such  as  transformer  connections 
and  design,  switches  and  switching,  protective  devices,  wave-form  of 
electrical  power  apparatus,  line  construction,  telephone  apparatus,  etc., 
may  be  found  in  textbooks,  proceedings  of  .societies  and  periodicals. 
Xo  attempt  has  been  made  to  include  such  references  here. 

It  will,  of  course,  be  understood  that  the  Committee  in  no  way 
expresses  opinion  respecting  statements  made  in  the  discussions  listed, 
by  referring  to  them  here. 

A  list  of  references  concerning  the  effect  of  transformers  on  the 
wave-form  of  currents  and  voltages,  as  influenced  by  magnetic  satura- 
tion, is  given  in  Technical  Report  No.  59;  and  a  list  of  references  on 
the  computation  of  electric  induction  in  Technical  Report  No.  64. 

Inductive  interference — General. 

Inductive  Disturbances   in  Telephone   Circuits.     J.   J.   Carty.     Trans    A.    I.    E.   E. 

Vol.  8,  p.  114,  1891. 

Transposition   and    Relative  Location  of  Power  and  Telephone  Wires.     P.  M.  Lin- 
coln.    Trans.  A.  I.  E.  E.  Vol.  21,  p.  245,  April,  1903. 
Transposition   of  Electrical  Conductors.     F.  F.  Towle.     Trans.  A.   I.   E.  E.  Vol.  '-'o, 

p.  i M!)?  1!M>4. 
Protection    of  Telephone   and   Telegraph    Lines   near   High   Tension    Linos.     It.    E. 

Hiefwood,  Jr.   Electrical   World  and  Engineer.  May  21,  1904. 
The    Influence    of    High    Tension    Conductors    on    Telephone    Lines.     V.    Schroltke. 

Elek.   /oil..   April    11),  1!)()T. 
Inductive   Disturbances    io   Telephone    Lines.     L.    Cohen.     Trans.    A.    I.    E.    E.    Vol. 

2<5.  p.   1  1  .V,.  May.  I'.IOT. 
ElVccfs   of    High   Tension    Power  Transmission   Upon  Telephone   Lines.     O.    Brauns. 

Elek.  Zeiu  April  9,  190S. 
Indnciivc    Interference    with    Telephone    Circuits    in    Proximity    to    High    Potential 

Transmission  Lines.     Electric  Review.     New  York.     June  13,  Unix. 
Inductive   Disturbances.     Mirabeli.     Journal   Telegraphique   de   Berne.     August   2,"> 

1909. 
Telegraph  and  Telephone  Systems  as  Affected  by  Alternating  Current  Lines.     J.  B. 

Taylor.     Trans.  A.  I.  E.  E.  Vol.  28,  p.  11G9,  October,  1909. 
Induct  ive  Effects  on  Telephone  and  Telegraph  of  High  Tension   System  of  Khenish. 

Prussia.     Arch.  Post.  &  Tel.  November,  1909.     Science  abstracts  "B."     January 

31,  1910. 
Telephone  Line  Protection  from  High  Tension  Transmission  Lines.     R.  W.  Krauss. 

Sib.  Journal  of  Engineering.      December,  1909. 
Inductive    Disturbances.     Muller.     Journal    Telegraphique    de    Berne.     March     2.~. 

1910. 
Telephone    Disturbances    from    Earl  lied    Three-Phase    Systems.     E.    Von    Holstein 

Ralhlou.     Elek.  Zeit.     June  23,  1910. 
Influence  of  High  Tension  Installations  on  Telegraph  and  Telephone  Installations. 

Electric  Review,  New  York.     June  30,  1910. 


70  FINAL   REPORT   ON  INDUCTIVE   INTERFERENCE 

Interference  Between  Energy  Transmission  Lines.     London  Electrician,  October  28, 

1910. 
A  Method  of  Preventing  Inductive  Troubles  in  Telegraphy.     G.  Girousse.     Comptes 

Rendus,  July  10,  1911. 
Influence  of  Heavy  Current  Lines  on  Light  Current  Lines  and  Apparatus.     Karl 

Hohage.     Elek.  Zeit.,  December  28,  1911. 
The    Maintenance    of   Telegraph    Lines.     C.    M.    Yorke.     Telephone    and    Telegraph 

Age,   1912, 

Inductive  Disturbance  on  Telephone  Lines.     B.  Smith.     Telephone  Engineer,  Jan- 
uary, 1912. 
Interference    between    Transmission    Lines    and    Telegraph    and    Telephone    Lines. 

Electrical  World,  February  24,  1912. 
Protection    of    Low    Tension    Lines    Against    High    Tension    Lines.     M.    Girousse. 

Bull.  Soc.  Int.  Electriciens,  June  12,  1912. 
Relation  of  Transmission  Lines  to  Telephone  and  Telegraph  Lines.     H.   B.   Gear. 

Electric  Review  and  Western  Electrician,  February  8,  1913. 
Disturbance  of  Telephone  Circuits  Due  to  3-Phase  Transmission  Lines.     O.  Brauns. 

Elek.  Zeit,  February  13,  1913. 
Telephone   Disturbances    from   Three-Phase   Lines.     O.    Brauns.     Electrical    World, 

February  22,  1913. 
Disturbance  to  Telephone  Circuits  from  Power  Lines  with  and  without  Grounding  of 

the  Generator  Neutral  Points.     Elek.  Zeit.,  May  22,  1913. 
Inductive  Disturbances  as  Affecting  Telephone  and  Telegraph  Lines.     P.  J.  Howe. 

Electrical  World,  May  31,  1913. 
Interference    with    Power    and    Telephone    Lines.     Electric    Review    and    Western 

Electrician,  June  28,  1913. 
Inductive  Effects  of  Traveling  Waves  on  Telephone  Lines.     K.  W.  Wagner.     Elek. 

Zeit.,  June  4,  1914. 
Investigation    of    Inductive    Interference.     F.    E.    Pernot.     Journal    of    Electricity, 

Power  and  Gas,  June  27,  1914. 
Neutralization  of   Inductive   Interference.     Journal  of  Electricity,  Power  and  Gas, 

November  21,  1914. 
Telephone    Transmission    in    its    Relation    to    High    Tension    Distribution.     E.    S. 

Moorer.     The  London  Electrician^  November  27,  1914. 

The  Successful  Operation  of  a  Telephone  System  Paralleling  a  High  Tension  Trans- 
mission Line.     C.  E.  Bennett.     General  Electric  Review,  December,  1914. 
Power    Circuit    Induction    with    Telegraph    and    Telephone.     S.    C.    Bartholomew. 

London  Electrician,  January  29,  1915. 
Power  Circuit  Interference  with  Telephone  Lines.     S.  C.  Bartholomew.     Telephone 

Engineer,  April,  1915. 
The  Inductive  Effects  of  a  140,000  Volt  Transmission  Line.     R.  D.  Parker.     The 

Michigan  Technic,  October,  1915. 
Inductive  Interference  Between  Power  Transmission  Circuits  and  Telephone  Lines. 

Trans.  A.  I.  E.  E.  Vol.  34,  p.  2113,  1915. 
How    a    Transmission    Company    Prevented    Telephone   Troubles.     F.    E.    Gillespie. 

Electrical  World,  December  11,  1915. 

Desirability  of  Transpositions  in  Power  Lines.     N.  E.  L.  A.  Bulletin,  April,  1910. 
Some  Problems  of  Inductive  Interference.     A.  H.  Griswold,  L.  P.  Ferris  and  R.  W. 

Mastick.     Journal  of  Electricity,  Power  and  Gas,  April  15,  1916.     Telephone 

Engineer,  May,  1916. 

Causes  of  Telephone  Noise  and  Its  Elimination.     Electrical  World,  June  17,  1916. 
Serving  and  Safeguarding  the  Public.     F.  C.  Dunbar,     Ohio  N.  E.  L.  A.  Monthly, 

July,  1916. 
Inductive  Interference  as  a  Practical  Problem.     A.  H.  Griswold  and  R.  W.  Mastick. 

Proc.  A.  I.  E.  E.,  September,  1916. 
Eliminating  Transmission  Line  Telephone  Troubles.     E.  P.  Peck.     Electrical  WTorld, 

September  9,  1916. 


FINAL    REPORT    ON   INDUCTIVE   INTERFERENCE  71 

Interference   UH  ween  Electric  Circuits.     Electrical  World,  December  1'.".,    11)10. 
Transmission  Lines  Interference.     L.  K.  Hurtz.     Telephony;  December  28,  1916. 
Inductive    Interference — Report    of    Committee    on    Overhead    Lines    and    Inductive 
Interference.     N.  E.  L.  A.,  June,  1917. 

Interference  from  electric  railways. 

.\eu(  ralizing  Inductive  Disturbances  in  Telegraph  Lines  Due  to  Single-Phase  Cur- 
rents.    Western  Electrician,  March  21,  1908. 
Inductive    Interference   from    Single-Phase   Railway   Circuits.     American   Telephone 

Journal,  June  27,  1908. 
Overcoming    Disturbances    of    Telegraph    Working    Caused    by    Electrical    Traction 

System.     C.    Mirabeli.     Post    Office    Electrical    Engineering    Journal,    January, 

1909. 
Disturbances    in   Telephone   Lines    from   Alternating   Current   Railways.     C.    Stein. 

Elek.  Zeit.,  August  15,  1912. 
Telephone  Disturbances  on   Electrical  Railway  Lines.     F.  Marguerre.     Elek.   Zeit., 

November  21,  1912. 

Minimizing  Induction  from  Single-Phase  Railways.  Electrical  World,  May  2,  1914. 
Inductive  Interference  With  Railway  Lines.  London  Electrician,  January  29,  1915. 
Influence  of  Alternating  Current  Railway  Lines  on  Telephone  Lines.  Telephone 

Engineer,  November,  1915. 
Single-Phase    Traction    and    Feeble    Current    Lines.     G.     Girousse.     La    Lumiere 

Electrique,  November  29,  1915. 

Publications  by  the  Joint  Committee  on    Inductive   Interference. 

Inductive  Interference  by  High  Tension  Lines.  Journal  of  Electricity,  Power  and 
Gas,  January  25,  1915. 

Proposed  Investigation  for  Inductive  Interference.  Journal  of  Electricity,  Power 
and  Gas,  March  15,  1913.  Western  Engineering,  April,  1913. 

General  Progress  Report  of  Committee  on  Inductive  Interference.  Journal  of 
Electricity,  Power  and  Gas,  October  4,  1913.  Western  Engineering,  Novem- 
ber, 1913. 

General  Progress  Report  of  Joint  Committee  on  Inductive  Interference.  Journal  of 
Electricity,  Power  and  Gas,  January  31,  1914,  Western  Engineering,  February, 
1914. 

Report  by  the  Joint  Committee  on  Inductive  Interference  to  the  Railroad-  Commis- 
sion of  the  State  of  California.  July  7,  1914.  Published  by  the  California  Rail- 
road Commission.  Trans.  A.  I.  E.  E.  Vol.  33,  p.  1441,  1914.  Journal  of 
Electricity,  Power  and  Gas,  September  12,  1914. 

Discussion  of  Irregular  Wave  Form.     Trans.  A.  I.  E.  E.  Vol.  34,  p.  1171,  1915. 

Progress  Report.  Trans.  A.  I.  E.  E.  Vol.  34,  p.  2113,  1915.  Journal  of  Electricity, 
Power  and  Gas,  September  25,  1915. 

Discussion  of  "Characteristics  of  Admittance  Type  of  Wave-Form  Standard." 
Bedell.  Trans.  A.  I.  E.  E.  Vol.  35,  p.  1711,  1916. 

State  Public  Utility  Commissions. 

California,  General  Order  No.  39,  August  20,  10.14.     Construction  and  ()|M'r:ition  of 

Power  and  Communication  Circuits  which  are  or  are  proposed  to  be  so  located 

as  to  create  a  parallel. 

Illinois,  General  Order  No.  30,  October  12,  1910.  Overhead  Electrical  Construction. 
Iowa,  Decision  and  Order,  December  30,  1910.  Electrical  Interference  between 

\4ransmission,  telephone  and  telegraph  lines 
Ai'.nKK^hvriONS : 

A.  I.  E.  E. — American  Institute  of  Electrical  Engineers. 

N.  E.  L.  A. — National  Electric  Light  Association. 

Elek.  Zeit.— Elektrotechnische  Zeitschrift. 


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